Why don't kolas and kangaroos exist anywhere else in the world but Australia? Why are they unique to Australia

I believe that millions of years ago, Australia was connected to the rest of the continents. But, it broke off of the rest of the continents first, so it became isolated from the rest of the world. This created a smaller gene pool relatively and allowed the evolution (if you believe in it) of marsupials that are unlike any other animal anywhere else.