User blog comment:Xilinoc/Ch. 639 - BABY,HOLD YOUR HAND 2/@comment-2211792-20150820080309/@comment-24531362-20150821195550

It is possible that Urahara was deemed capable of controlling him. Then, when Urahara was banished, it is possible that Mayuri's usefulness and/or sheer power prevented and/or precluded his reimprisonment.