I don't know. And fail in who's eyes? If you asked people on the board, Gerrard Warren was a failure, yet he had a long starting career in both Cleveland and a few other teams.
Lot's of reasons why guys don't make it. I think a big one is some players goal is to get drafted high, hit their lottery, then coast the rest of the way. Their goal was met once they signed the contract. There are no more goals after that. That's human nature. Just reading this board you see people saying if they made what players made for just one year they could retire. I am not putting that down, just saying to illustrate the point.
Sometimes it is injury. Sometimes just plain poor draft evaluation. I guess it has to be one of those 3 things.