User blog comment:B214/Volume 22 spoilers/@comment-27083298-20161115021134/@comment-1359322-20161116120609

I can provide you with something simpler.

"Unlike Devils who choose by strength regardless of species, Heaven chooses and reincarnates those who are strong followers and who are very talented as their main targets."

- Volume 13, Extra Life

And there's a proper reason for that. Remember Volume 18, when Ise was at the Heaven? If they randomly choose people and in large scale, there's a chance that it may affect the Heaven's system entirely.