I think there are a lot of people including NBA GMs that thought the Bucks would be much better this year.
I'm guessing it has to go back to defense. Redd is no defensive ace and Mo Williams has athleticism and quickness but little defensive discipline. Desmond Mason is solid defensively but at 6-5 even with long arms he has a size disadvantage on most nights. Both Villanueva and Bogut are decent rebounders, but neither is very good defensively. And, I would imagine that collectively, they are even worse when it comes to team defense.
And, maybe it's a lack of iden y. I think Mo Williams and Charlie Villanueva fit better in an uptempo fast break type team, while Michael Redd and Andrew Bogut are more traditional half court type players. Perhaps they struggle with iden y there.
Those are just two guesses. But, they really should have been better this season.