Oh, this is great. There may have been more results since I was working on a field project studying them, but to my knowledge we have absolutely no idea! They are not particularly well adapted to the cold, but their range keeps extending northward. This well predates the rapid climate change caused by humans, so we cannot use that as a reason. They are a bit of a mystery.
My guess would be that they are occupying a niche where limited brain and limb development (problems all marsupials face) are not limiting factors on success. Maybe their lack of a close genetic relation when surrounded by placental mammals gives them some pathogen resistance when scavenging? Those are just mildly educated guesses. When I was working with them we had no idea, and our field results were not at all enlightening.