Board Thread:General Discussion/@comment-24970687-20140531231739/@comment-24282510-20140601003539

While i see three reasons.

He didn't need it. Remember Azien himself stated that Yamamoto's shikai alone was far to powerful for him to beat head on. These is farther backed up by Azien making/modding up Wonderweiss.

Aziens power made him think think he was in bankai.

They were in the world of the living. They just switched the real town with the fake. Even with the barrier i doubt the barrier could keep Yamamoto's bankai power in.