Marsupials are pouched animals. Almost all marsupials live in Australia. Only one kind of marsupial lives in the United States: the possum.
All marsupials in Australia are descended from animals that came to Australia over 50 million years ago. After all of those animals "boarded onto" Australia, the continent separated from the rest of the continents. The animals couldn't go onto another area, and other animals couldn't come into Australia.