You can go to BC and see massive valleys of farmland (maybe used for grazing) that could easily be built on.
90% of Canadians live within 100 miles of the US border because: 1) the weather is better and 2) transportation options 150 years ago drove settlement (transCanadian railway and access to waterways) and 3) development begets more development, so established communities tend to grow rather than new one sprouting up.
The area in the red is one continuous granite slab called the Canadian Shield. The US has no comparable geography.
There's almost zero topsoil, and building any community there involves a lot of very expensive blasting. It's bad land for the opposite reason that the Australian continent is largely uninhabitable: there's tons of water, but no soil.
(The only real upside to it is that it's very conducive to hydroelectric energy; the vast majority of Canadian energy production can be, and has always been, zero-emissions- we call electric companies "the hydro", and combined with the nuclear reactors in Ontario, 85% of national electrical power generation is zero-emissions.)
Superimpose a population density map on the Eastern provinces, and you'll notice that generally speaking, the settled area of the provinces ends where the shield begins. That isn't a coincidence.
Sure, climate change should make a bit of the [non-red-shaded] land more economically useful, but that's about it, and mining/farming communities generally aren't large enough to provide a base for a new city especially because it's landlocked- so you're not going to see any new cities spring up there any time soon.
I think you're just ascribing cause when in fact northern Alberta is just as empty but has no Canadian Shield.
I was going to say, there is still a lot of land both outside and inside the “Shield”.
If you want a house and a job in this country, you have to go to Alberta, Saskatchewan, or Manitoba. BC has no land and is either mountain or agricultural land reserve (the second-largest city in the province has massive fields in the middle of it; average single family home price $1M). ON and QC are similarly boxed in for land (the Canadian Shield is very difficult to build on, and if you don't know French the only part of Quebec you can really go to is the Montreal area), and the Maritime provinces have land but no work.
The important thing to remember is that, unlike the US, the vast majority of Canadian land is much more difficult to inhabit- 90% of the population lives within 100 miles of the border, because that's where the good land ends.