It simply means that the governments of the two countries haven't arranged this yet. Often visa-free is bilateral - especially when done with a trade deal, but sometimes not. For example, Aussies can get a tourist visa for the UK on arrival, but UK citizens need one (an ETA) in advance to visit Australia! (at least, last I checked).
By default, most countries require you to have a visa. Then as agreements are struck up, or they see it as financially viable to allow tourists in (cough Bhutan cough), they may start making it easier or cheaper for those specific groups to enter.
Often it's to nurture growth and relationships with neighbouring countries. For example, Mongolia has offered visa-free access to a few of the 'stan countries since the 1990s!
Interestingly, Mongolia grants visa free access (of varying lengths) to 46 countries and/or territories.
Comparing the visa policy of Mongolia and these 46 countries with the page on visa requirements for Mongolian citizens you can see how much of it is reciprocal!
So it's more a case of not why Australians need visas, but why Australia hasn't yet gotten to a more open relationship with Mongolia. Perhaps they're focusing on other countries first. But the good news is the world is opening up, and it's more and more likely to change. Keep the fingers crossed