Why are there a lot of endemics in Australia?

Endemism is characteristic of Australia for the reason that this continent has been in isolation for more than a hundred million years, respectively, animals and plants evolved without the influence of living organisms on other continents. This is the reason that most of the marsupials survived here, while on other continents (except for some species in South America) they became extinct and gave way to placental mammals. For the same reason, many endemic gymnosperms have survived here.