Sure, but they didn't fail due to a lack of population. I still don't get your point. Canada has a higher population to a sports team than does the states. Simply Canada is under-represented in terms of population. If all you were looking at is population Canada would have more teams not less.
From what I read the problems for the Expos were a terrible owner, the attendance was lackluster, and the city shut the down the funding to new a stadium and used the taxpayers money elsewhere. Developed talent was difficult to keep (which oilers fans know to well). The Grizzlies also suffered similar issues and declining attendance. I'm saying that a larger population has more of chance to fill those seats, when you have more people that may have interest in the sport. Plus the NBA and MLB also got involved and it just wasn't looking good all around.
Higher population will garner a higher surplus of taxes to spend on something extra like a stadium (if the a city approves it), but Montreal used it for the hospitals instead.
Look how much the City of Edmonton had to dish out for Rogers place.
Total tax revenue for U.S for a fiscal year is about 3.18 trillion. Canada from reports since 2015 is 28.2 billion. This difference is population.
I'm not saying the canadian cities can't handle major sports franchises, I'm saying because of the low population, it is more difficult to sustain it unless you have unprecedented interest. Edmonton has been a below average team for the past decade and yet the support still remains encouragingly strong.
More teams would be awesome, but someone has to pay for them.