Australia is amazing simply for the fact that it's so isolated. Usually environments are surrounded by other different ones but Australia is completely cut off from the mainland. This means that for millions of years these animals adapted and evolved to be the master of their domain, without some invasive species taking over the majority of the biomass.
It's like if you have a nice garden that you isolate from everything, you can grow a lot of cool stuff. If it isn't isolated you have to worry about weeds coming in and taking over until you have nothing but those weeds, and none of your cool stuff.