All of those provinces have vast swathes of undeveloped land. The Canadian Shield is not hard to build on, plenty of cities are built directly on bedrock.

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.