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, they were never granted the ability to be complete hybrids.

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