Well, think about it -- good anime films aren't made in an echo-chamber. You want people with different experiences in different films (be it animated, life-action, 3d, etc). A non-anime director has experience outside of anime. Hence, they're able to look at anime (with its tropes, genres, and story-telling styles) with a new perspective.
That's what you want -- new perspectives.
This isn't new: an experienced director dips their toes in different types of genres, movies, and mediums. Overtime, they become known for a certain type: however, that comes with experiencing different things.
Does that clear things up?