There's a reason Ilyasova just got traded for Sean Williams and the ghost of Caron Butler... He hasn't been healthy EVER. Every time the Bucks started to find a rhythm with him in the lineup he immediately got injured.
Milwaukee would rather have two guys that won't challenge anyone for playing time than have his inevitable injuries derail their team chemistry.
Well that and the fact that getting rid of his contract for two guys who have unguaranteed contracts that they are going to waive will save them more than enough money to re-sign their immensely talented young players (Like Khris Middleton) AND go after big name free agents this summer (Like Brook Lopez and Tyson Chandler).