Why are there so many marsupials in Australia and New Zealand compared to elsewhere?

Here is a good rundown of what is known about marsupial development.. They slightly predate placental mammals, and inhabited the southern regions of the supercontinent Pangaea before it began to split into Antarctica, Australia, New Zealand, and the smaller islands versus Asia about 100 million years ago. The oldest marsupial fossils are found in modern China, and date to about 125 million years ago. Most of the marsupials in Australia, New Zealand, New Guinea and the smaller islands were isolated there when that landmass split from Antarctica. I am not sure whether conditions there favored marsupials over placental mammals, or they just happened to be the dominant species there upon isolation. Can anyone help fill in my blanks on why marsupials ended up succeeding so well in the Australian island complex?