Canadians love living in Canada and in general move to the US very reluctantly. As a result Canadians have fewer career options relative to their US counterparts. This results in low turnover. You're right, it has nothing to do with a company's "pro-employee" which I assume means "employee-friendly" ways.
I would bet that most Canadians, even those working in the USA, feel that the USA is a bit messed up on some issues and has an arguably lower quality of life. But it's got an economy that's an order of magnitude bigger and opportunities to match.
I think that is true in some fields, like the performing arts and untrue in others like engineering esp. telecommunications where top talent is homegrown thanks to the likes of Nortel and now RIM. These people do not want to move and there is a critical mass that hasn't gone away even after the bubble burst.