Could it not be as simple as: all the gods who are *different* alignments want that soul as well? We've already seen Thor and Loki going in to dispute the assignment of a bunch of souls to Hel, and while they were explicitly doing that in order to throw a spanner in her plans, there must be an agreement between the Gods that allows that sort of thing to happen or else they'd be getting in trouble. So, the Gods want to be sure that any souls that get assigned to them are legitimately theirs to make sure no-one else comes along and grabs them later.