Mammals on Australia Are Mostly Pouched (Marsupials) and the Mammals

Mammals on Australia are mostly pouched (marsupials) and the mammals elsewhere in the world are mostly not pouched. Why is this?

A) A secure pouch is necessary for the highly variable and chaotic Australian habitats and not so much elsewhere.
B) Pouched mammals have only ever existed on Australia ,and it's too far to migrate.
C) No one knows why this is. It's an area of biodiversity under intense study.
D) The continent of Australia has been isolated from the others such that its species have evolved separately for a long time.
E) Australia is closest to Antarctica and there are no mammals there to migrate and create a population of non-pouched mammals.