I'm sorry, but that sounds like the author (of the canon DxD) was trying to make Azazel sound cool and different without bothering to give a good reason. Don't get me wrong, I still like the novel, but there was no reason for Azazel not to reincarnate humans. They were the smallest in number, all while being the least trusted. That's enough reason to rebuild their numbers.

She's his mother. Then again, this is an anime fanfiction after all.

and she won't be the only one.

Yes, but romance will not be the main focus.