Question: Why did the Apollo landing crew decide to begin preparations for the EVA early?
About the article "Apollo 11", section "Landing".

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After separating from the Command/Service Module, the Lunar Module "Challenger" and its crew of two, Eugene Cernan and Harrison Schmitt, adjusted their orbit and began preparations for the descent to Taurus-Littrow.
While Cernan and Schmitt prepared for landing, Command Module Pilot Ron Evans remained in orbit to take observations, perform experiments and await the return of his crew-mates a few days later.
Soon after completing their preparations for landing, Cernan and Schmitt began their descent to the Taurus-Littrow valley on the lunar surface.
Several minutes after the descent phase was initiated, the Lunar Module pitched over, giving the crew their first look at the landing site during the descent phase and allowing Cernan to guide the spacecraft to a desirable landing target while Schmitt provided data from the flight computer essential for landing.
The LM touched down on the lunar surface at 2:55 PM EST on December 11.
Shortly thereafter, the two astronauts began re-configuring the LM for their stay on the surface and began preparations for the first moonwalk of the mission, or EVA-1. 
Most of the first part of the day after arriving in lunar orbit on July 30 was spent in preparing the Lunar Module for descent to the lunar surface later on that day.
When preparations were complete, un-docking from the CSM was attempted; it did not occur, because of a faulty seal in the hatch mechanism.
The Command Module Pilot, Alfred Worden, re-sealed the hatch; the LM then separated from the CSM.
David Scott and James Irwin continued preparations for the descent while Worden remained in the CSM, returning to a higher orbit to perform lunar observations and await his crewmates' return a few days later.
Soon, Scott and Irwin began the descent to the Hadley landing site.
Several minutes after descent was initiated, at pitch-over and the beginning of the approach phase of the landing, the LM was six kilometers east of the pre-selected landing target.
On learning this, Scott altered the flight path of the LM.
They touched down at 22:16:29 UTC on July 30 at Hadley, within a few hundred meters of the planned landing site.
While previous crews had exited the Lunar Module shortly after landing, the crew of Apollo 15 elected to spend the rest of the day inside the LM, waiting until the next day to perform the first of three EVAs, or moonwalks, in order to preserve their sleep rhythm on a mission on which they were to spend a significantly longer time on the surface than previous crews had spent.
Before they slept, Scott performed a stand-up EVA, during which the LM was depressurized and he photographed their surroundings from the top docking hatch. 
The crew continued preparing for Lunar Module activation and undocking shortly after waking up to begin flight day five.
The boom that extended the mass spectrometer out from the Command/Service Module's Scientific Instruments Bay was stuck in a semi-deployed position.
It was decided that Young and Duke would visually inspect the boom after undocking from the CSM in the LM.
They entered the LM for activation and checkout of the spacecraft's systems.
Despite entering the LM 40 minutes ahead of schedule, they completed preparations only 10 minutes early due to numerous delays in the process.
With the preparations finished, they undocked in the LM "Orion" from Mattingly in the Command/Service Module "Casper" 96 hours, 13 minutes, 13 seconds into the mission.
For the rest of the two crafts' passes over the near side of the Moon, Mattingly prepared to shift "Casper" to a circular orbit while Young and Duke prepared "Orion" for the descent to the lunar surface.
At this point, during tests of the CSM's steerable rocket engine in preparation for the burn to modify the craft's orbit, a malfunction occurred in the engine's backup system.
According to mission rules, "Orion" would have then re-docked with "Casper", in case Mission Control decided to abort the landing and use the Lunar Module's engines for the return trip to Earth.
After several hours of analysis, however, mission controllers determined that the malfunction could be worked around and Young and Duke could proceed with the landing.
As a result of this, powered descent to the lunar surface began about six hours behind schedule.
Because of the delay, Young and Duke began their descent to the surface at an altitude higher than that of any previous mission, at .
At an altitude of about , Young was able to view the landing site in its entirety.
Throttle-down of the LM's landing engine occurred on time and the spacecraft tilted forward to its landing orientation at an altitude of .
The LM landed north and west of the planned landing site at 104 hours, 29 minutes, and 35 seconds into the mission, at 2:23:35 UTC on April 21.
After landing, Young and Duke began powering down some of the LM's systems to conserve battery power.
Upon completing their initial adjustments, the pair configured "Orion" for their three-day stay on the lunar surface, removed their space suits and took initial geological observations of the immediate landing site.
They then settled down for their first meal on the surface.
After eating, they configured the cabin for their first sleep period on the Moon.
The landing delay caused by the malfunction in the Command/Service Module's main engine necessitated significant modifications to the mission schedule.
Apollo 16 would spend one less day in lunar orbit after surface exploration had been completed to afford the crew contingency time to compensate for any further problems and to conserve expendables.
In order to improve Young's and Duke's sleep schedule, the third and final moonwalk of the mission was trimmed from seven hours to five.
The next morning, flight day five, Young and Duke ate breakfast and began preparations for the first extra-vehicular activity (EVA), or moonwalk.
After the pair donned and pressurized their space suits and depressurized the Lunar Module cabin, Young climbed out onto the "porch" of the LM, a small platform above the ladder.
Duke handed Young a jettison bag full of trash to dispose of on the surface.
Young then lowered the equipment transfer bag (ETB), containing equipment for use during the EVA, to the surface.
Young descended the ladder and, upon setting foot on the lunar surface, became the ninth human to walk on the Moon.
Upon stepping onto the surface, Young expressed his sentiments about being there: "There you are: Mysterious and Unknown Descartes.
Highland plains.
Apollo 16 is gonna change your image.
I'm sure glad they got ol' Brer Rabbit, here, back in the briar patch where he belongs."
Duke soon descended the ladder and joined Young on the surface, becoming the tenth and youngest human to walk on the Moon, at age 36.
After setting foot on the lunar surface, Duke expressed his excitement, commenting: "Fantastic!
Oh, that first foot on the lunar surface is super, Tony!"
The pair's first task of the moonwalk was to unload the Lunar Roving Vehicle, the Far Ultraviolet Camera/Spectrograph (UVC), and other equipment, from the Lunar Module.
This was done without problems.
On first driving the lunar rover, Young discovered that the rear steering was not working.
He alerted Mission Control to the problem before setting up the television camera and planting the flag of the United States with Duke.
The day's next task was to deploy the Apollo Lunar Surface Experiments Package (ALSEP); while they were parking the lunar rover, on which the TV camera was mounted, to observe the deployment, the rear steering began functioning without explanation.
While deploying a heat-flow experiment that had burned up with the Lunar Module "Aquarius" on Apollo 13 and had been attempted without success on Apollo 15, a cable was inadvertently snapped after getting caught around Young's foot.
After ALSEP deployment, they collected samples in the vicinity.
About four hours after the beginning of EVA-1, they mounted the lunar rover and drove to the first geologic stop, Plum Crater, a crater on the rim of Flag Crater, a crater across.
There, at a distance of from the LM, they sampled material from the vicinity of Flag Crater, which scientists believed penetrated through the upper regolith layer to the underlying Cayley Formation.
It was there that Young retrieved, at the request of Mission Control, the largest rock returned by an Apollo mission, a breccia nicknamed Big Muley after mission geology principal investigator William R. Muehlberger.
The next stop of the day was Buster Crater, about from the LM.
There, Duke took pictures of Stone Mountain and South Ray Crater while Young deployed a magnetic field experiment.
At that point, scientists began to reconsider their pre-mission hypothesis that Descartes had been the setting of ancient volcanic activity, as the two astronauts had yet to find any volcanic material.
Following their stop at Buster, Young did a demonstration drive of the lunar rover while Duke filmed with a 16 mm movie camera.
After completing more tasks at the ALSEP, they returned to the LM to close out the moonwalk.
They reentered the LM 7 hours, 6 minutes, and 56 seconds after the start of the EVA.
Once inside, they pressurized the LM cabin, went through a half-hour briefing with scientists in Mission Control, and configured the cabin for the sleep period.
Shortly after waking up on the morning of flight day six three and a half minutes early, they discussed with Mission Control in Houston the day's timeline of events.
The second lunar excursion's primary objective was to visit Stone Mountain to climb up the slope of about 20 degrees to reach a cluster of five craters known as "Cinco Craters."
After preparations for the day's moonwalk were completed, the astronauts climbed out of the Lunar Module.
After departing the immediate landing site in the lunar rover, they arrived at the day's first destination, the Cinco Craters, from the LM.
At above the valley floor, the pair were at the highest elevation above the LM of any Apollo mission.
After marveling at the view from the side of Stone Mountain, which Duke described as "spectacular," the astronauts gathered samples in the vicinity.
After spending 54 minutes on the slope, they climbed aboard the lunar rover en route to the day's second stop, station five, a crater across.
There, they hoped to find Descartes material that had not been contaminated by ejecta from South Ray Crater, a large crater south of the landing site.
The samples they collected there, although their origin is still not certain, are, according to geologist Don Wilhelms, "a reasonable bet to be Descartes."
The next stop, station six, was a blocky crater, where the astronauts believed they could sample the Cayley Formation as evidenced by the firmer soil found there.
Bypassing station seven to save time, they arrived at station eight on the lower flank of Stone Mountain, where they sampled material on a ray from South Ray Crater for about an hour.
There, they collected black and white breccias and smaller, crystalline rocks rich in plagioclase.
At station nine, an area known as the "Vacant Lot," which was believed to be free of ejecta from South Ray, they spent about 40 minutes gathering samples.
Twenty-five minutes after departing station nine, they arrived at the final stop of the day, halfway between the ALSEP site and the LM.
There, they dug a double core and conducted several penetrometer tests along a line stretching east of the ALSEP.
At the request of Young and Duke, the moonwalk was extended by ten minutes.
After returning to the LM to wrap up the second lunar excursion, they climbed back inside the landing craft's cabin, sealing and pressurizing the interior after 7 hours, 23 minutes, and 26 seconds of EVA time, breaking a record that had been set on Apollo 15.
After eating a meal and proceeding with a debriefing on the day's activities with Mission Control, they reconfigured the LM cabin and prepared for the sleep period.
Flight day seven was their third and final day on the lunar surface, returning to orbit to rejoin Mattingly in the Command/Service Module following the day's moonwalk.
During the third and final lunar excursion, they were to explore North Ray Crater, the largest of any of the craters any Apollo expedition had visited.
After exiting "Orion", the pair drove the lunar rover away from the LM before adjusting their heading to travel to North Ray Crater.
The drive was smoother than that of the previous day, as the craters were more shallow and boulders were less abundant north of the immediate landing site.
Boulders gradually became larger and more abundant as they approached North Ray in the lunar rover.
Upon arriving at the rim of North Ray Crater, they were away from the LM.
After their arrival, the duo took photographs of the wide and deep crater.
They visited a large boulder, taller than a four-story building, which became known as 'House Rock'.
Samples obtained from this boulder delivered the final blow to the pre-mission volcanic hypothesis, proving it incorrect.
House Rock had numerous bullet hole-like marks where micrometeoroids from space had impacted the rock.
About 1 hour and 22 minutes after arriving, they departed for station 13, a large boulder field about from North Ray.
On the way, they set a lunar speed record, traveling at an estimated downhill.
They arrived at a high boulder, which they called 'Shadow Rock'.
Here, they sampled permanently shadowed soil.
During this time, Mattingly was preparing the Command/Service Module in anticipation their return approximately six hours later.
After three hours and six minutes, they returned to the LM, where they completed several experiments and offloaded the rover.
A short distance from the LM, Duke placed a photograph of his family and a United States Air Force commemorative medallion on the surface.
Young drove the rover to a point about east of the LM, known as the 'VIP site,' so its television camera, controlled remotely by Mission Control, could observe Apollo 16's liftoff from the Moon.
They then reentered the LM after a 5 hour and 40 minute final excursion.
After pressurizing the LM cabin, the crew began preparing to return to lunar orbit. 
  January 31 - Apollo program: Astronauts aboard Apollo 14 lift off for a mission to the moon.
February 5 - Apollo 14 lands on the Moon.
February 9 - Apollo program: Apollo 14 returns to Earth after the third manned moon landing.
May 19 - Mars probe program: Mars 2 is launched by the Soviet Union.
May 30 - Mariner program: Mariner 9 is launched toward Mars.
June 30 - The crew of the Soyuz 11 spacecraft are killed when their air supply leaks out through a faulty valve during re-entry preparations, the only human deaths to occur outside Earth's atmosphere.
July 26 - Apollo program: Launch of Apollo 15.
On July 31 the Apollo 15 astronauts become the first to ride in a lunar rover a day after landing on the surface.
November 13 - Mariner program: Mariner 9 enters Mars orbit.  
Deke Slayton was the Director of Flight Crew Operations and effectively chose the crews for the flights.
He did not intend to give astronauts two lunar landing commands but, according to historian Michael Cassutt, as late as the summer of 1969—when 10 landings were still scheduled—Slayton planned to give Lunar Module Pilots Fred Haise, Edgar Mitchell, and James Irwin the opportunity to walk again on the Moon as Commanders.
During the early Apollo missions he used a rotation system of assigning a crew as backup and then, three missions later, as the prime crew; however, by the later Apollo flights, this system was used less frequently as astronauts left the program, Slayton wanted to give rookies a chance, and astronauts did not want to take backup positions that no longer could lead to prime-crew spots.
In the case of Apollo 18 the crew was probably the Apollo 15 backup crew:  Richard F. Gordon Jr. (Commander (CDR))  Vance D. Brand (Command Module Pilot (CMP))  Harrison Schmitt (Lunar Module Pilot (LMP)) When Apollo 18 was canceled, Schmitt was moved up to Apollo 17 under pressure from the scientific community, replacing Joe Engle.
Schmitt, a geologist, became the only professional scientist and the twelfth man to walk on the Moon.
Slayton's intention for the Apollo 19 crew was the original (prior to cancellation) Apollo 16 backup crew:  Fred Haise (CDR)  William R. Pogue (CMP)  Gerald P. Carr (LMP) For Apollo 20 there is even more uncertainty.
Based on normal crew rotation, the crew would likely have been:  Stuart Roosa (CDR) (replacing Pete Conrad, already CDR on Apollo 12)  Paul J. Weitz (CMP)  Jack R. Lousma (LMP) Another possibility would have been:  Stuart Roosa or Edgar Mitchell (CDR)  Jack R. Lousma (CMP)  Don L. Lind (LMP)