As this map shows, there is little difference in population density between adjacent regions of Canada and the USA. The geography and climate of the USA includes a lot more land capable of supporting dense populations.

In short. It's cold. Most of the land area is inhospitable for a lot of the year, and they can't grow anything or support livestock so far north. Like siberia.

I worked for a very large Canadian Corporation from Winnipeg once. It was explained to me that its colder in Canada in general. Like -50 to -40 deg below in the winter in Winnipeg and that is a Large City. So Cold that planes don't fly as it is to cold. Also the engines can freeze quickly if they shut off after they land at cold temps.