>>508592094 (OP)Because its extremely inhospitable. Deathly cold in the winter, swampy and wet in the spring and summer with mosquitos and other pests, impossible to farm because soil is sparse and low quality and cash crops don't really survive there. Its a very unstable environment for humans.
If you go further north, like the arctic circle and Alaska, the native snow niggers survive on plentiful Arctic animals like seals and the pest issue more or less goes away. The coastal areas are more settled due to access to fisheries and connection to shipping lanes for the transport of raw materials like lumber and ore for trade, but there is zero reason to live in the Canadian shield region because it offers very little. Many of the workers that rely on those industries are seasonal and go elsewhere during the winter.
In some ways, its sort of like the Amazon rainforest, which is probably the most dangerous environment on Earth. I'm an environmental engineer and geologist - have done surveying and mapping work in parts of the world where you get airdropped via helicopter because no roads go there, so I know a little bit about survival. Of all places on earth, the only one where I wouldn't want to go is the Amazon rainforest. Even the natives there don't venture too deep into the forest away from the Amazon River. You will be swarmed by pests and horrible parasites, and its so humid that your clothes will literally rot off you. Most of the Amazon has never been mapped or observed by humans - we only know about it due to remote sensing via satellites.
Cities have been strategically places by the initial pioneers of those regions to be viable for economic function. There is nothing economically viable about areas like the Canadian shield beyond very small scale, specialized use cases.