User blog comment:Tekkphreke/Theory on Ukitake's Bankai/@comment-3143776-20130912154203/@comment-5147109-20130912160958

2 potential things on that, 1. His body is relatively weak so he can't sustain his fighting abilities for too long which would level out that amount of power, and 2. If you'll recall he actually had been knocked out of the battle by Wonderweiss before Aizen came into the picture.