Australia was covered in Gondwanan rainforest for millions of years after its split from that great southern supercontinent, but as it drifted north, climatic changes, specifically drying and increased seasonality, a new drought adapted flora emerged and spread until it covered most of the continent.  Relicts of the Gondwanan rainforests clung only to the eastern coastline, where wetter conditions enabled it to persist (White, 1986).
A clade of marsupials evolved to take advantage of the new grasslands, and some of these found a niche in the rocky outcrops, becoming rock-wallabies.
Around 15 million years ago, Australia's northward drift brought it into collision with Asia and Malesian flora invaded the tropical lowlands, aided by birds and bats, including much of the habitat of rock-wallabies.  In this new flora they found an abundant and nutritional food source, and likely pressed their already-good climbing abilities into the service of accessing what the new trees had to offer (Martin, 2005).

In terms of tree-kangaroo evolution, the ancestry of rock-wallabies is quite noteworthy.  The karyotype, that is, the arrangement of chromosomes, of the Proserpine Rock Wallaby (Petrogale persephone) has been found to be basal to all other species (Eldridge and Close, 1994).
This is interesting as P. persephone lives within closed forest and seems equally happy climbing trees as bounding among the rocks, illustrating the remarkable similarity between the life-history traits of rock-wallabies and tree-kangaroos.

Proserpine rock-wallaby.  Source: http://matthewsyres.com/ Retrieved 8/5/15

The presence of Wallace's line (a deep ocean channel between the islands of Lombok and Bali in Indonesia), while not presenting a major barrier for plant dispersal, did stop the movement of fauna largely in their tracks (Wallace, 1869).  The absence of large folivorous and/or frugivorous creatures such as monkeys,  meant that the Malesian flora in Australia initially went largely unmolested.  Nature abhors a vacuum, as Aristotle once noted, and here were niches waiting to be filled.  Australian fauna quite happily took the baton, as has been observed in the New Guinea lowlands (Martin, 2005).  The role of large folivore is occupied by leaf-monkeys north of Wallace's line (Brandon-Jones, 1998) and it seems probable that rock-wallabies simply filled this niche.

Dusky leaf-monkey.  Source: Wikipedia.org Retrieved 8/5/15

References

Brandon-Jones, D. (1998). Pre-glacial Bornean primate impoverishment and Wallace’s line. Biogeography and geological evolution of SE Asia, 393-404.

Eldridge, M. D. B., & Close, R. L. (1994). Chromosomes and evolution in rock-wallabies, Petrogale. Australian Mammalogy, 19, 123-136.

Martin, R. (2005).  Tree-kangaroos of  Australia and New Guinea.  CSIRO Publishing, Melbourne.

Wallace, A.R. (1962). The Malay Archipelago. Dover Publications Inc, New York (unabridged republication of 1869 Macmillan and Company, London edition)

White, M. E. (1986). Greening of Gondwana: The 400 million year story of Australia's plants. Reed Australia.