Not an expert but my suspicion: fresh water, or lack thereof.

Australia's so damned flat that most moisture-laden air just flows right over it without dropping any rain.

The few significant hills and mountains that cause the humid air to churn and precipitation to form and fall are in the southeast. Which is thus where the biggest rivers are, and thus where the most people are.

On the north coast, the rivers seem few and far between, and pretty small, too.

Imma go with heat, it's hot as hell up there. Also, maybe we didn't want to talk to the rest of you lot, we're too busy having barbies and skulking back some cold beers.