Why should they? It makes sense to go to the metric/decimal system because it is the most logical, easy to use, and wide spread in the world. It affects every area of life, including weight, currency, distance, tools, manufacturing, etc. . . .
But Australia is a big continent, there is no reason for them to change the side of the road they drive on, just because others do. England would actually have more reason to change from left side of the road to right side of the road, because many tourists from continental Europe travel with their cars to the UK (especially since the Euro tunnel exists). But there is still no reason for Australia to make this move.
A better question would be why are the United States still using the antiquated system (lbs and inches or miles)? And why are they trying to teach the decimal system in the schools in a way that complicates everything (i.e., instead of comparing the meter to the yard, why not just show kids what a meter is and then divide that length in ten, hundred, thousand, so it is clear to the kids, instead of having them memorize that a meter is 1.2 or whatever yard).
By the way, if the United States were building cars under the metric system, they might have a better chance to compete in the world market with their cars (that is, as long as they would build cars that are more fuel efficient!)