As far as I know, in research involving live animals the standards for their care have also been raised significantly, so every animal sourced for the study needs to be housed and cared for properly. Additionally, after the study is over every one of those snakes needs to be placed in a proper home with a new owner - this is great for the animals but probably adds a burden in terms of how many can be used. It's not the case that someone can go out and purchase 300 corn snakes and simply euthanize all of them when they're finished - that sort of thing doesn't fly, at least at universities/labs I know of.