For the same reason they never reached South Australia either.
Didn't reach Queensland, either. In fact, we have the narrowest rail gauge I have ever seen - a good 8 inches less than the NSW gauge, modeled on the old English imperial gauge - which makes a mockery of the Qld gov'ts much touted high speed tilt train. Nope, a just on 4 foot gauge is not wide enough for a high speed train that's just as wide as standard rail carriages... more likely to come off on bends.
Stupid, really, that Oz states each have their own rail gauge, rather than making them uniforn across the country. I worked with Qld Rail some years back, and when it was decided that NSW trains would not terminate just South of Brisbane but go all the way to Roma Street Station in central Brisbane, we had to replace all the sleepers with wider ones and lay an extra rail to accomodate their standard gauge trains.
I imagine other states face the same issue... but it's going to take a politician with balls of steel the legislate for an Oz-wide standard rail gauge, given the cost due to the size of the place.... not to mention opposition from petty pollies demanding their state keep the gauge unique to it. Sure it'd be a costly exercise, but in the long run, Oz would save trillions with a universal standard rail gauge.
Not gonna hold my breath but.... can't expect politicians to see the bigger picture, can we!