Yea weather, but also the Canadian shield. There is just a thin layer of soil on top of the bedrock over a huge portion of Canada.

Further south is warmer in winter

But I think a lot of it is economic. Port cities are where money changes hands, and we've effectively smeared them all along the boarder.

Also this map is kinda misleading. Edmonton and the entire 4 maritime provinces are not here, and the main reason so much of the population is close to the border is that something like 1/3 of the population is clustered around the great lakes/st Laurence, because people came here by boat.

I'd argue the weather as the biggest one, at least for BC. Northern BC is absolutely beautiful but it's cold af in the winter and burny as hell in the summer. If we're talking far north in the territories, I know another issue is infrastructure because it's much more difficult to build/get stuff up there. Though this meme misses a big part of the Indigenous/Inuit population that lives up there.

Also, this country is just fucking huge, like bigger than I think any of us realize. If our population were to spread out, it would be a very thin spread.

Edit: I kept thinking about this and got help with some math on it. If the population of Canada were to spread out, we'd each have about 64 acres (0.256km) to ourselves. By comparison, the same calculation in the US with the US population would equal approx 0.03 km, or just over 7 acres per person.