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 Japanese empire was territorialy expanding into China. It's longterm goal was to colonize and eradicate the ethnic Chinese. The invasion of Manchuria is what many consider the signal start of ww2
