Well MDV3100 in early stage trials (at a higher dose) had a couple of seizures reported as SAEs. Even a low seizure incidence might seriously hamper its commercial prospects compared with Zytiga - not saying it should do so given the know long-term adverse effects of steroids, but I think it would.
Note the steroid dose for Zytiga is pretty small, but I think it is enough to make doctors think twice if they have a comparable steroid-free alternative.