Its a good question.
I don't think he is a good defensive player. He never has been. So, that always puts someone behind the 8-ball with Doc. I also think the lack of practice has really hurt his ability to get a rhythm offensively, and learn the system.
But more than anything, I think Doc is just rolling with the hot hand. KG, JO, and Bass are pretty clearly the top of the pecking order (and they are the best big men on the team by a large margin IMO), so its all about that 4th spot...and Steimsma took advantage of his opportunity.
I think Doc is a fan of guys earning their playing time, and Stiemsma has. At some point, he will probably slow down, or someone will get injured, and Wilcox can slide right in there. But right now, I think it has less to do with Wilcox as it does with Steimsma playing solid basketball.