Board Thread:General Discussion/@comment-4310945-20131229044249/@comment-26420452-20140104064152

Sternritter M, your first hypothesis seems to be the right one. At the time, Aizen only had an imperfect Hōgyoku. So, the Visored never got the capacity to become full hybrids.

Also, we should take into account Tōsen's desire not to be a Shinigami.