85% of Australia's population is urban, almost all of that in the capital cities of Sydney, Melbourne, Brisbane, Adelaide, and Perth. And there is, normally, a lot of interstate traffic, so those urbations are well connected. When we've slipped, COVID spreads through Australian populations very well indeed.
One of the first things done was to break those connections and set up hard borders between the states, so that each state, and to some extent each city, could concentrate on cleaning itself up, without worrying about whether more cases were being brought in from outside. Once a city or a state was clean, they kept the borders closed so that they stayed clean. Melbourne got hit hard, and our reaction was to lock down hard, and long. No unnecessary travel. Strict definitions of what "necessary" is, and if visiting your dying relatives if you're not actively a carer for them is not necessary, then neither is a haircut. Curfews. 5km travel limits. Compulsory masks outside the home. And that was kept up for months, until we started seeing double doughnuts: no new local cases, no deaths. Then we relaxed, carefully, and started accepting travel. And when there were new outbreaks, which we knew there would be, we locked down again.
The difference between Australia and the US? We passed the marshmallow test.
The marshmallow test is something given to small children to measure their maturity and self-control. You put them in a room, with a marshmallow on the table. (Or a chocolate, or some other kiddy-crack treat.) And you tell them that they can't have it now, but if they don't touch it, then in five minutes they can have two. Then the experimenter leaves the room. Young children can't control themselves: they see a thing they want now, so they take it now. There is a certain level of maturity where the child realises that for putting up with a bit of temptation, they can get a bigger reward, so they wait. Sometimes they literally sit on their hands to stop themselves taking the marshmallow, but they wait. And after five minutes, they get two marshmallows.
Australia and New Zealand are enjoying our two marshmallows.
America had to keep having haircuts. And couldn't close unnecessary retail. Had to have sit-down restaurant meals instead of take-out. Couldn't put up with masks. Weren't able to lock down interstate travel, much less inter-city. You, as a society, couldn't stop yourself having the marshmallow in front of you. And now you don't get a second one.