I feel that it partially may have come down to the railroads and, more importantly, distributing the less populated northern areas to those who can exploit the land. And, looking it up, so the Métis and First Nations could be kept out of the way while Ottawa worked to bring in millions of immigrants (explicitly white, trying to keep any others out) with which to settle the land and bring in loads of cash. And to then send everything directly to be exported by Ontario, Quebec, etc as free trade and lower tariffs with the US was treated like treason for a while. I do remember once seeing a map on an old Map Thread about a Manifest Destiny involving going Pole to Pole. All the Canadian Praire Provinces have their borders going up, slicing through Yukon, Northwest Territories, and Nunavut.
It partially works in real life. Straight lines going north, and keeping along a long from Alaska to the Hudson Bay. The biggest issues are when straight lines are put between already highly populated areas. Plus sometimes t is best to not use the obvious geographic features as the border. I can only imagine how New Mexico would work if the Rio Grande was used as a border between states. Dante are would be in some economic trouble. Maybe not the best example.