User blog comment:Salubri/The Thousand-Year Blood War arc Plot Points/@comment-24234990-20131130191339/@comment-11051098-20131130194046

I don't know. It seems to be something like that.

He went to the world of the living the 2nd time without requesting permission, and gave up his Shinigami-powers to save the life of a human. Both were illegal, and the latter could possibly have been punished with execution. It's basically the same as Rukia did when she transferred her powers to Ichigo - Isshin transferred his powers into Masaki's soul to stabilize it.

I suppose he didn't take the chance to go back to SS and risk getting captured and punished.