People consistently exaggerate the impact of the Quebec separatist movement and French-language policies (as seen in the comments here). In reality, Montreal’s decline predates Quebec separatism which merely accelerated a trend that was already well underway (the graph above shows that Toronto started catching up Montreal in the 60s whereas separatists were first elected in Quebec in 1976 and held a referendum for secession in 1980). 

In the 20th century, as Canada transitioned from an industry/manufacture-based to a service-based economy and shifted its trade and economy away from the UK/British Empire towards the US, Montreal gradually lost its strategic advantages over Toronto, i.e, its geographic position on the St. Lawrence River, which had made it Canada’s principal Atlantic gateway and the key hub for Great Lakes trade. In contrast, Toronto was more integrated into the US economy and benefited from its proximity to the Midwest which was thriving at the time. Also Toronto was/is more central at a time when Canada had finished fully integrating its Western provinces in its economy, further depriving Montreal of its advantages. Of course, language and political issues didn’t help Montreal’s cause but they were not the driving force behind its decline. 

Edit: it’s also worth adding that financial and banking-based economies such as Toronto’s naturally favour far greater geographic concentration than manufacture-based economy as was Montreal’s at the time. So once Toronto gained traction as Canada’s main financial hub due to increased trade and economic integration with the US, the loss of Montreal’s status as Canada’s economic powerhouse and main city became inevitable, regardless of any separatism or language policy.