User blog comment:B214/Volume 22 spoilers/@comment-104.230.254.245-20160819121246/@comment-9595671-20160820012707

Actually, I think I figured out why Gabriel was chosen to run Heaven in Michael's place. In Ajuka's case, he stayed because he was one most necessary for Devil's survival due to the Evil Pieces. However for the Angels, Michael said it's the faith of those on Earth is necessary.

So the one who leads them likely depends on how kind or forgiving they are so they can properly continue Gods teachings. Gabriel from what I've seen is just as nice if not nicer than her brother is, so that could be why she was chosen to stay behind.

In fact, it's possible that Michael even believed she was a better fit for the Angels' leader than him. Nothing says he took over the leadership for the Angels willingly when God died after all, but he likely had to for the sake of his people.