Power tripping isn't really about being "tempted by power", rather it's when you become self-righteous because you got granted (some) power.
If you want some sweeping generalisation for why Americans are more prone to this sin, how about the various old cultural patterns of thought that fit under the header "manifest destiny"?
Australia is a colonial nation that unified its entire continent - that's beating the US, who have two large neighbors and a couple small ones.
(Of course both countries have indigenous nations inside them. They're actually very similar, which I think is the reason Australians tend to be culturally resentful of the US on surveys in a way almost no other country is.)
If you want some sweeping generalisation for why Americans are more prone to this sin, how about the various old cultural patterns of thought that fit under the header "manifest destiny"?