Why does the australian outback receive so little rainfall?

It's flat. Very flat. And big, too, with a dry, hot center, perfect for forming stationary blocking highs and upper level troughs. Basically, nothing gets through from west to east without a rare and special authorization, and the once-moist air stagnates and dries out in a baking hot oven. Rain has to sneak its way past, usually by scraping along the southern edge, or by coming in from the north or north-east.