Northern Territories Monsoon Firing Up

I've been trying to puzzle out the strange phenomena of Australian weather, and one odd feature is why doesn't the annual NT monsoon extend the full breadth of the continent? Why is Australia so dry in the middle? Why isn't it like the U.S., where the southwestern monsoon can sometimes extend along the Sierra Madre and the Rocky Mountains, all the way to the Canadian border? Is the absence of the mountain ranges a key feature? But at least its raining in some places now (just not everywhere, like down south, where the aridity is unparalleled). The northern crocodiles will be most appreciative!