[bombcar]

It's expensive and difficult to put a large door on the front or back of a plane, resulting in inefficiencies or maintenance headaches.

The planes that DO have them are almost always military planes not used for other things, or very large jumbos (search 747 cargo or C-5 galaxy).

Almost all other planes are optimized for passengers (the cargo space in even a largish jet may be too short to stand up in, for example).