It's a barren wasteland.

To speak specifically to the Alaska situation, geography, economics, and politics all play a role.

Geographically, people in Alaska live along the coasts. The ocean acts as a moderator, helping to keep the coastal areas relatively comfortable. Anchorage has about half of the Alaskan population and averages a high of 78 in the summer to a low of 5 in the winter. Barrow, on the other hand, ranges from 47 to -19 - a noticeable difference.

Tied to geography is economics. Yukon is landlocked, save for the Arctic coast, while Alaska is all coastline. The fishing industry in Alaska is a huge source of jobs. Another source of jobs is oil and gas. Alaska has a very well developed oil industry with the famous Trans-Alaskan pipeline. The Yukon doesn't have this same natural wealth. They do have shale oil, but that's an industry that has only been developed within the past 20 years and it's quite unstable due to price fluctuations. have Tied to economics is politics. The US pays people to live in Alaska through the Alaska Permanent Fund. Taxes are very low because the state uses the oil revenues instead and extra money is returned to the people who live there. The US has always had an interest in maintaining a population in Alaska. Maintaining a population is a way of exercising control over land (see Texas and the US-Mexican War for a good example of why) and after purchasing the land from Russia, there was a concerted effort to build and maintain a population there.

As for Canada as a whole...

75% of Canadians live within 100 miles of the US border. The rest of Canada, though beautiful, is cold and barren. Besides hunting, trapping, or logging, there isn't much else to do out there. So as far as demographics are concerned, Canada is basically a 100 mile wide, 3000 mile long country, while the US is able to use the entire lower 48. If you look at growth rates, growth in the US southwest is explosive - Texas, Arizona, California - while places like Montana, Idaho, and North Dakota are anemic.

If you look at the actual numbers though, you'll see that Canada has always grown steadily and in fact from 1941 to 1951 the country grew 22.1%, and from '51-'61 it grew 30%. The US by contrast only grew 14.5% and 18.5% respectively. Canada actually had the larger baby boom than the US did per capita.

TLDR - US government incentivizes people to live in Alaska. Alaska has more economic opportunity. Canada is cold and empty. Most US growth is in the warm and sunny climates.