I believe it was because the AF motor was inside the camera body and not in the lens like the EF lenses are now.
Current DSLR Nikon's are backward compatible with all Nikkor lenses but certain bodies (Like the D7000/D7100) can AF older lenses while others cannot. (Like the D5300)
At this point in the game, I think Canon would have a munity on their if they changed the camera lens mount again. I know I wouldn't stick around.
Interesting. I haven't really researched the AF tech available back then for the FD equipment.
Guess I lucked out choosing an EOS 620 way back when...