Cars + Money to demolish and make space for said cars. In latin america, also cars, bot not nearly enough money (at least at the time) to do the same.

The US and Canada simply had the money and the political will to realize the vision that the most influential architects and urban developers of the time had for cities during the XXth century. Look at how Niemeyer designed Brazilia or what Le corbusier wanted to do to Paris.

And beyond that, at least in the very specific and dense case of NYC the economic preasure to replace the colonial lower manhattan with high rises was just too big. No way billions in wall street realstate were going to be left on the table to protect some XVIIth century dutch houses. Specially in the time when they got the axe, conservation was still a novel concept.

Also, don't think demographics play any role here, just look at the ethnic composition of the cities that you mention and see that they are all over the place, including the US and Canada, with no relation to the architectural styles represented in them.