It will be hard to get startups to be based in a Canada if their target market is the US. We’re talking a difference of 36 million people vs 300 million. When it comes down to it Canada looks big but is a really tiny country.
Which is not to discount California's tremendous housing crunch. I am merely pointing out that its a much lower risk for a young single US citizen to just move to CA, try it out for a few months, if they can't get a job or make it, then just drive back. It is much harder for people from outside the country to do the same.
Bit pedantic but this really is a far cry from being the "vast majority" of Canada's population.