It's that simple. In addition, few people are successfully trolled like BYU fans. They are hypersensitive. They have a chip on their shoulder. That have a wonderful football history that leads them to vehemently defend their team. They have a fairly passionate fan base. And there is the obvious religious divide between the two schools. There are plenty more examples as well.
I don't have anything to back this up, but I feel that very little of the trolling has to do with people not getting into BYU. I think most of the people who troll BYU have no interest in that type of college experience.
As for the trolls, they relish in stirring people up and getting a reaction. This place is like a petri dish for trolls.