I would guess it's because Australia is, like, 99% uninhabited and uninhabitable or borderline uninhabitable, 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.
american exceptionalism at its finest. :)
My state, about the same land area as France has a population density of 39/sq-km, right at the median value for the states. France has density of 122/sq-km. Population density is lower here, where people actually live, than in many locations in Europe or Australia. This may, in part, explain public transportation issues in the USA.