Maybe the destroyer of the Worldstone (Tyrael) decides and moderates the flow of demons and angels onto Sanctuary. And maybe Tyrael's plan was to destroy the Worldstone to gain that power to use it for the cause of evil - that's why the Heavens are not fond of his decisions.
If Diablo III ends the single-player franchise, so be it, but I hope they give an MMO for it so that the true fans can play Diablo for even longer and pay their respects for the franchise monthly.
I think people have unanimously agreed on this one. Wide open