Well a couple reasons I have noticed.
First, my snake would not touch pinkies/fuzzies, but went right after a hopper, pinkies and smaller end fuzzies might not seem like "food" since the snake is instinctively "programmed" to go after mice, and this is some reason why my snake would only take a hopper.
Secondly, some snakes will only take one mouse at a time. Again I have tried to feed my snake multiple mice but he took one and was done, some snakes don't like to eat more than one rodent per feeding. Another reason is the size of the snake's digestive system. 2+ pinkies/fuzzies are much longer than one hopper, and the snakes stomach would not be able to handle that long of a meal, and could result in regurgitation.