It's pretty simple. There have been far fewer cases and deaths than in the US. As for the vaccine rollout, it is largely a function of the early success in controlling infections.
The initial vaccine doses destined for Australia were diverted to more urgent areas (which was the right thing to do). Then as domestic product of AZ ramped up, a scare campaign ramped up (media love to make people worried) and the body for recommending vaccines gave advice for under 50s to not take AZ (whilst there was no actual alternative). Because covid lockdowns were effective, and Australians were seeing such low rates, there was complacency amongst the population and they were willing to wait for their vaccine of choice. Now delta is looking like it is making lockdowns less effective, and there is ample supply of Pfizer, people are scrambling to get vaccinated.
The initial vaccine doses destined for Australia were diverted to more urgent areas (which was the right thing to do). Then as domestic product of AZ ramped up, a scare campaign ramped up (media love to make people worried) and the body for recommending vaccines gave advice for under 50s to not take AZ (whilst there was no actual alternative). Because covid lockdowns were effective, and Australians were seeing such low rates, there was complacency amongst the population and they were willing to wait for their vaccine of choice. Now delta is looking like it is making lockdowns less effective, and there is ample supply of Pfizer, people are scrambling to get vaccinated.