User blog comment:B214/Question and Answers 6/@comment-114.30.107.245-20160829151156/@comment-9595671-20160829165118

The Brave Saints are still not as widespread as Evil Pieces, so keeping Dulio behind make sense as otherwise Heaven would have lost too many of it's strong fighters.

This is probably why Azazel was the only member of the Fallen Angels that got sealed away, they couldn't afford to send anyone else to help fight Trihexa since they lack an equivalent reincarnation system.