I would guess it's because Australia is, like, 99% uninhabited and uninhabitable or borderline uninhabitable[1], which probably means they can focus resources almost exclusively on the few densely populated areas. Additionally Australia's urban population seems to be significantly higher than America's (~89% vs ~80% according to 5 seconds of Googling I just did) so there's probably greater political will for investing in urban areas, and that political will probably also isn't resisted/sabotaged by an electoral system that grants hugely disproportionate representation to non-urban areas that have little to gain from realistic/economically sensible investments in public transportation.
[1] https://www.reddit.com/r/MapPorn/comments/8ehplw/australian_...