Often, the whole story, including all the numerous subplots has to exist within eight pages. Similarly, a feature film must warm up its audience within 20 minutes of the title sequence. This translates to approximately twenty script pages. The short film doesn't have this luxury. Often, the complete back-story and rough elements of characterisation must be revealed within two pages.
Economy is essential.
They know when you try to indoctrinate them in a hurry, and they don't like it. That scene where one neighbour turns to the other and describes the prior years of the film has to go. Very few films put a flashback to good purpose. Sometimes there is very little choice, particularly in short films. However, be very careful, and make sure that all other alternatives have been exhausted first.