Most houses in Australia are single storey because that is how it has been since time in memorial. This style of building was introduced in the nineteenth century from United Kingdom. Most of these single storey houses are found in the suburbs. However, there are house that have more than one storey still found in Australia. Read more on http://en.wikipedia.org/wiki/Terraced_houses_in_Australia.
Because kangaroos don't do stairs well? Honestly, I have no idea. It may be just a coincidence, or it may be something logical. Here in the US, in many states where it would be a really good idea to have a basement in the event of tornadoes, it is impractical to dig a basement due to the hardpan. Delouise or any of my other Australian friends around to address this one?