Politics, mostly. WA & QLD governments have upcoming elections & are showing themselves to be "putting x state's people first" or whatever phrase they're using. Never mind the fact that (relatively small) tourist spots like Port Douglas are in an economic hole, the masses of votes are in Perth & Brisbane.
SA has started to look at opening borders, because, well, no upcoming election
NT has the added whammy of vulnerable remote communities and less of a tourism driven economy, which could explain their hesitance.