It could make more sense to bucket these three together if you're looking for what they have in common.

  Australia     14.5t 
  United States 14.3t
  Canada        14.0t

My guesses are: houses rather than apartments, driving everywhere, percentage of SUVS compared to sedans, meat consumption, general consumerism?

In the US, we also have large numbers of homes that have not been brought up to modern efficiency standards and cheap/outdated, grossly inefficient heating/cooling contributing. That number could probably be brought down quite significantly without negatively impacting quality of life by “simply” (I’m aware it’s a huge undertaking) properly insulating homes and in urban/suburban areas banning heating/cooling solutions below a certain efficiency threshold.

The tragedy of of this is that these are improvements that would actually improve life in these houses - making them healthier, more comfortable. Trouble is, retrofit is expensive.

I'm Canadian most of our emissions this past year was because of forrest fires.

That is balanced by not having to heat.

The more likely explanation for the first two is that plenty of fossil fuels are available so they are used inefficiently.

Probably mining and refining natural gas and oil?

In that case there is so much to win by improving the mining/refining processes.

I generated this list a few months ago. I picked a threshold population (I think 10 million) and listed the top 5 and then some other groups. I think I would also guess that resource rich countries spend a lot on cars and AC.

FYI, I edited list with latest numbers after your comment.

AC is pretty efficient and the temperature differential it needs to overcome is smaller than winter heating in most places. For these places specifically it seems to obviously be the production of oil for the first two and coal for the third. The availability of fossil fuels tends to make them cheaper and consequently a lot more is used.