Too many posters are concentrating on the back story here...was it this, was it that.
The bottom line, and I mean bottom line is that IF Riley had been #2, he would have stayed. There is nothing else here to concentrate on, to speculate about, etc., other than he knew that he was beaten out by Dobbs. If he had beaten out Dobbs, he'd still be here.
If you've played, you know that feeling that you get when the coach has made his decision, and it's not you. It's the other guy. Riley felt that feeling, didn't think he could change Butch's mind, and decided he'd be better off at another school.
It's just that simple.