I love Theis. Smart player, hard worker, standup guy. I don't think I agree that Williams was ready for much bigger minutes last year but this season, by the all star break, Williams had made it clear he was the best center on the team but Stevens wasn't giving him comparable minutes to Theis.
It may have been loyalty, wanting to reward hard work, or wanting to force some of the mistakes out of Williams' game by keeping him on a short leash... There were lots of possible reasons Brad leaned more on Theis but I also think a possibility is that we sometimes look at the numbers (in this case, the advanced numbers) and talk ourselves into players being something that the eye test doesn't bear out.
Like I said, I love Theis but I think his limitations (of size of talent) put his ceiling at something below starting center for a champion.