Because, while he has scored some, he hasn't been able to rebound or shown the ability to do the little things defensively. Particularly in the last game, when he came in the defense suffered and the offense was not smooth (could be more to do with guard play).
Here is description for him. He is a talented scorer, but a wild horse. He does not have the feel to play in team concepts, especially on defense. He is great in pickup ball. If he raises his IQ and plays with his team, he could be a good role player (ala Mike Scott).