Heh, I'm Australian and so have seen the cuteness many times, and up close :)

I didn't know they'd gone in that direction though! I learnt that marsupials have basically no brains compared to their placental mammal counterparts but I always assumed that the causation went something like:

Very little food in Australian bush -> animals which already had low energy requirements (by being dumb) thrived -> no pressure to change because the environment was boring

Let me know if you think you have an answer to the bigger brains question! I recently read a linguistics book about the origins of language, and there was a lot of historical background on our savannah ancestry and how we might have come to need language in the first place. But the author handwaved the brains part completely, just noting that in his scenario where we were scavenging dead mammoths and whatnot, the increased protein and fat would have let us grow bigger brains - nothing about why.