The answer is obvious when you realize that in any given year 5% of people generate 50% of the costs. Also, you can expect 70% to 75% of the people in a given year will not have a need to access the health care system. That is why health care polling doesn't work.
Let me clarify that. I should say "in a serious way based on claims dollars". That is, they will not have face any serious impediments to service due to the minor nature of their claims. On average, around 25% of the people in a given year will have no claims (i.e. require no medical services). The problem, of course, is that the 25% who do need serious service in a given year is not the same 25% as last year.
Secondly, not all Canadians have used the American health care system so they just don't know. The fact that some are able to state they prefer the American to the Canadian system is a demonstration that some Canadians have, in fact, used our system. Those folks found the Canadian system lacking.
My sister in law's sister was invited by the Canadian Defense Force to provide a demonstration of a new type of ski fitting. Next thing you know she had appendicitis while in Canada. The CDF was unable to "schedule" her for a medical appointment of any kind. In the end they used a Canadian government helicopter to fly her to Seattle where she could be seen immediately.
That's not necessarily "anecdotal" ~ it reflects the systemic bias against foreigners built into the Canadian health care system. The place is functionally not a safe place for Americans to travel because they will not tend to you, and you cannot otherwise "purchase" medical care up there. As far as Canada is concerned a foreigner can just die in the street when it comes to even emergency care.