I think one factor with American cities that doesn't really apply to Canadian cities is the phenomenon of white flight, which would aggravate the trend to suburbanization. Plus Canadians never really embraced the automobile as much as Americans.
Very true. Even in the US, the cities held up as champions of urban vitality (e.g. Seattle, Portland, San Fran, Minneapolis) never had the African American population and resulting issues around white-flight.
Toronto's downtown also benefited from some seemingly random, or at least unplanned, developments.
1.) Downtown has several post secondary institutions (UofT, Ryerson, OCAD, GBC). These bring tons of people into downtown every day. In most US states, much more of this happens in college towns. Those towns, not surprisingly, happen to have lots of the livability that Toronto prides itself on.
2.) Big government presence. Most American states don't have their capital in their largest city. The net result is to put lots of high-paying, white collar jobs in smaller state capitals. Toronto's also a bit unique in that QP is actually a very large government by North American standards. Amongst sub-national governments, Ontario's probably just behind California, New York and Texas in terms of total spending and well ahead of similarly sized states like Ohio or Illinois.
3.) The country's capital markets happen to be here, and for some reason that's one of the few industries where firms are willing to pay premium rents for central office space (transactional efficiencies from being close?). Not every city can have one and most of the activity in the US has been consolidated in Manhattan and Chicago.
That's hardly an exhaustive list. Torontonians can be a little self congratulatory on the vitality of downtown vis a vis the US and attribute that success to seemingly random things (e.g. lack of highways). If you adjusted for those three industries, and the support services they rely on, Toronto's CBD employment share would be pretty close to any other US city.