When I look at that list of players who have left from Sampson and Capel, a few things stand out.
1) Many who left never succeeded playing basketball at any level after leaving OU. (exceptions: Lav, McKenzie, etc.)
2) Many who left were not disciplined enough to play college basketball and to be a student-athlete.
3) Some who left were dealing with family issues and needed to be closer to home.
4) Many who left were just mentally weak and quit on themselves and their team because they either weren't getting the playing time they thought they deserved, they weren't doing as well on the court as they were used to their entire lives, or the coaches actually yelled at them and preached fundamentals, unlike AAU.
Maybe I'm wrong, but the current generation of AAU, And1 basketball players are incredibly undisciplined and very poorly coached from Day 1 of their basketball playing lives. Fundamentals are a thing of the past with these guys. They've been the star athlete their entire lives, then they get to college where everyone is as athletic as they are, and they can't handle it mentally. They aren't disciplined enough to wake up at 6am every day to get in the weight room. They aren't discplined enough to stay at the gym until midnight working on their game. Obviously, there are exceptions, and those are the guys who become great basketball players (Blake Griffin)
And before you go blaming the coaches for recruiting these types of kids, I'll say this. It's very easy for a kid during recruitment to tell Capel or any coach recruiting him that he has the drive and desire to be the best, that he'll live in the gym, he'll keep his grades high, etc. It's another to actually do it.
Of course, Capel has to be checking with their teachers, coaches, parents, etc. to try to get a better idea if the athlete will live up to his promises to you, and I'm sure he's doing that. However, there's only so much you can do. Recruiting is a guessing game then hope like hell it turns out well. There are very, very few sure things.