You may put your pants on one leg at a time, but millions of people are putting their pants on simultaneously. Programs should have an abundance of natural parallelism to exploit. On a parallel computer of 100 processors most people quickly notice that a program is not running 100 times faster. Most people have little or no experience with parallel computing, and there are few useful parallel programs to use off-the shelf or even good samples to copy from. People developing parallel systems software are similarly behind on their learning curves.
Parallel programs port poorly.
Someone may do a decent job on one machine, but when the program is ported to a new machine it is so different that drastic changes need to be made just to permit the program to be run. Most serial computers have the same basic organization, but this is not so for parallel computers. Some parallel computers, such as the currently popular Beowulf systems, are essentially just a collection of computers linked together with Ethernet. They use simple commands, similar to read and write, to communicate among the processors. Such parallel computers are known as message-passing systems, or distributed memory computers.