Why mount speakers in the wall instead of the ceiling for a surround sound system?

The ideal location(if the room allows) for a pair of rear surround speakers in a home theater for the best effects is the side walls, 3-4 ft above the listener's head(when seated), and no less than 18" from the corner. The reason is because the rear speakers should produce diffused sound. In other words you want more reflected sound than direct sound coming from the rears. You should never aim them directly at the listener because it will make the speakers easy to localize with the ear and this will hinder the "surround" effect. Only the front speakers should be aimed at the listener. Placing the rears on the ceiling means you would get more direct sound from them. The 2nd best spot for the rears is the rear wall. And the least desireable spot is the ceiling.