Its likely the isolation, its why we have so many marsupial variants (possum, kangaroo, wombat, koala, etc) as well as monotremes (platypus, echidna). We also until relatively recently (human migration by the indigenous australians ~50 000-65 000 years ago) had megafauna, Im talking about marsupials the size of Hippos wandering the land and predators to match them, both went extinct as the herbivores were killed and the predators starved having been specialized to hunt the large game and less adapted by chasing and eating smaller nimbler prey.

The isolation allows for niche animals to further fulfill the specific niche with less generalized organisms to compete against (there are a couple migratory birds, but the system is more or less enclosed). Why specifically poison? The evolutionary arms race idea is often put forward but I reckon a big factor is how desolate a large portion of Australia is. Its dry and barren which encourages less active chasing prey system and more reliant on specialized protein systems to make the kill (venom) where ambush predation is the least taxing on the animal.