User blog comment:Xilinoc/Ch. 659 - There Will Be Frost/@comment-1248963-20160121181832/@comment-5398287-20160121234745

iirc the Bankai Hollowfication was only a temporary ordeal - I imagine because introducing Hollow Reiatsu to an intrinsic part of a Shinigami's soul for an extended period of time without the Hogyoku there to stabilize it would have some messy results.