I think that age was a big factor in him not being drafted. However I think the biggest reason was that he isn't known for being the superstar at any one thing. He can do most things well. He is a good shooter but he doesn't light up the gym on shot after shot. He plays good defense but isn't a shutdown guy. So on and so forth. He will never be a "star" in the NBA but he could be very very useful as he also has no major weaknesses. And he will continually cause people to underestimate him as he can run, jump and has pretty good body control. (and leaving him open can be a major mistake). And leaving him open on a drive can be an even bigger mistake as he is a serious dunker.
I think the reason people got a little down on him at Pitt as they expected him to be Paul Pierce in his prime and he was never going to be a superstar. Just a solid player.
I get irritated when all the people want a young player to be Michael Jordan and get overly down on him when he isn't. It causes a lot of older players to be underated.