What is the selective advantage of being a marsupial?

Question

I.e., what would be the disadvantage of being a placental mammal in Australia?

Answer 1

+ Shorter gestation period

+ Don't have leave young behind

+ Can keep young safe from predators

+ Young are always protected by parents

+ Young can be fed quickly

Answer 2

Pregnancy doesn't last nearly as long, which would in turn provide better chance for survival for the mother as she wouldn't be AS vulnerable as a placental. Australia is a continent that normally requires many animals to constantly be on the move to find food and water. Having a developing baby in your pouch can provide a little more mobility for the mother where as a placental mother would be slowed down while searching for food/water/escaping from predators in the vast lands of Australia.