It's just a hunch, but I believe it is because the space on each side of the cage is limited, and often the feeders are installed on front of the cages. Depending upon the size of the feeders you mount on your cages, it eats up a significant amount of space.
Most of my cages measure 24" x 24", while doe cages measure 30" x 24". 12" are what the actual door space measures in each instances. On the doe cages, there's obviously a little more room to work with, but with the 24"x24", that door accounts for better than half the space across the front of the cage by the time you figure in the overlaps created by door guards. If I'm using a 5.5" feeder, that accounts for pretty cramped space on the front, and it's why I have resorted to installing feeders on the doors. For me, it just makes better use of what limited space I have in each cage.