Weather will be the primary reason. Despite its huge size. the real habitable area is probably less than 10% of its geographical size. Even Southern Ontario is deemed by many I know to be "too cold to live". 90% of Ontario and Quebec are simply inhabitable. I also personally think the entire Manitoba and Saskachewan are not suitable for human habitation unless we can create a way to simply hibernate during the 6 month winter (why there are so few people in Montana and North Dakota?) The fact that Montreal are thrive as a vibrant metropolis is already sort of a miracle and we don't see many such large cities with such severe weather. The only other case I can think of is probably Moscow.
The radiation left after the war. Along with the many people who died thereafter for many years. This also caused a nuclear war of sorts later on with Russia.
