They arrived in a land becoming increasingly dry. It was also old, eroded and infertile. Weathering leached nutrients from the soil over eons of geological stability. It had been a tough place to live for a very long time. This was the only place where the marsupials won the evolutionistic race with placentals.
Marsupials simply perform better in tough conditions but the cold-blooded animals did even better.
The very conditions favouring marsupials over placental mammals also favour ants, termites and curiously, frogs, in particular lizards, and of course snakes. Australia s first elapids came from a fertile, prolific Southeast Asia. Here they prospered on a diet of small mammals, birds and fish. Therefore, while there was plenty on the menu for the new arrivals it was a bit different to what they had encountered before. Rick Shine has shown most Australian snakes eat lizards.