Why is Australia mostly desert?

[Ascension]

Because it's on the Tropic of Capricorn. The worst deserts are on this line and the Tropic of Cancer. Sure there are exceptions like the Gobi and the US southwest but these areas are the way they are because of low precipitation while the ones on the tropic lines are the way that they are because of the angle to the sun making them hotter. At least, this is what I remember from school...I'm sure someone will pop up in a bit with a wiki link. If you want this place to be a desert and it's not on a tropic line then put a wall of mountains in the west that kind of wraps around a bit to the north and south. Otherwise it will get a lot of rain from storms.

[Karro]

I think Ascension is right about the tropics, except for one point: it's not because of the sun baking things (although that's what makes it a HOT desert), but it's still because of rainfall... Basically, because of the Coriolis Effect, wind patterns are driving moisture to and away from certain places. In the case of the Tropics, wind patterns are driving moisture away from those lines and toward either the Equatorial region (where we see a lot of hot, steamy rainforests) or toward the Temperate regions above and below the Tropics.

The Gobi and the Desert Southwest are caused by Rainshadow.