Waste and the associated expense thereof, for one. Especially for professional use - there is an average waste of half of the last roll of film on every shoot,

Waste and the associated expense thereof, for one. Especially for professional use - there is an average waste of half of the last roll of film on every shoot, and that means 8-12 wasted frames on 220 versus 4-6 on 120. For the consumer side, a very large percentage of cameras used a window on the backing paper for frame counts, something that would be disastrous for 220. And for anything that used a pressure plate rather than just film tension to maintain film flatness, you'd need to have an adjustable plate (many cameras didn't have that).