Due to continental drift,

All the continents were earlier together as a single landmass called 'Pangea'. They are broken into pieces due to certain geological changes like movements of crustal plates below the surface of the earth. This process of separation of  pieces of landmass is called continental drift. 

Due to this continental drift South America joined North America. The animals like hippopotamus and horse of South America were overridden by animals of North America. Australia got separated from other continents, its pouched mammals were prevented from facing any threat or competition from the mammals of other continents and hence survived and diversified on Australian land in a better way.