To answer the original question, the Trans-Canada Highway's original alignment is part of a compromise. According to A Road for Canada: The Illustrated Story of the Trans‑Canada (Daniel Francis, 2006), when the Trans-Canada Highway alignment was being determined, it was originally envisioned as a single coast-to-coast highway, as opposed to the multiple routes that exist today. The more direct all-Canadian route from Quebec to Western Canada (preferred by TCH planners) was to go through Ottawa and Northern Ontario, bypassing the more-populated Southern Ontario; however, the Ontario provincial preferred a route that passed further south along the St. Lawrence River. A compromise was reached where it would follow what is now known as the "Central Ontario Route" and "Georgian Route" between Ottawa and Sudbury (Hwy 17 between Sudbury and Ottawa was added to TCH later on).
In reality, it served neither route, except for the fine folks of Peterborough, especially when Hwy 401 was built. If ever there could be a section of highway that could become the "Historical Trans-Canada Highway", that could probably be it. It would make more sense to reroute it to follow Hwys 400 & 401 through Toronto.