It's not just Nebraska kids. Remember how Omar Manning panned out? Some of these guys just got too comfortable dominating kids in high school. College is a completely different game. It's why the recruiting rankings are such a sham most of the time.
And some of these talented kids aren't used to getting coached.
I agree completely regarding recruiting rankings. If you want to sign a four star kid who plays as hard as a "walk-on", get in line. Everybody will be after those guys. Other 4 star kids get offered and then not much becomes of it. Wonder why?