Different history of colonization policy in Quebec and Ontario. Colonization in Ontario was shut down in the 1930s during the Great Depression. In Quebec, formal colonization was more tightly integrated with the Church, had more institutional support, and officially continued until 1973. There were still government-backed homesteading projects in the 1960s in Quebec. Also, on the Ontario side in the early 20th century there was no road/rail connection except via Quebec. Which meant that development in the region was tied more to Quebec than southern Ontario. And Ontario had little reason to support that. So it remained government land on the Ontario side. Or at least that's how I understand it.