Here are the most common reasons I have heard:

1. Some students are so concerned about their own GPA that they won't share notes or study with others. They're worried that helping someone else will hurt them. I don't know how widespread this is, but I've heard it enough times from different sources that i believe it. In contrast, the culture at BYUI is much more collaborative.

2. Nerdiness. I don't know how to say it better than that, but as the average GPA and ACT scores for acceptance have trended higher, there's an overall increase in nerdiness that can make it difficult for some to socialize. I have heard this one less often, but enough that it's not just one person's opinion.

This is not meant in any way to paint all BYU students with a broad brush. I'm just making the point that BYU isn't for everyone - even for kids that were accepted there.

But go ahead and demean BYUI if you feel you need to. Says far more about you than it does about BYUI or Rexburg