Difficult question to answer since it's so ambiguous. One explanation that might contribute to that idea is the general character of Ontario, which is where I suspect this stereotype is derived. Up until the 1960s, Ontario was fairly British, Protestant and Conservative. As in, you couldn't drink on Sunday, and you bought liquor from the LCBO in brown bags so as not to 'offend' people. This culture produced Ontarians that were (stereotypically) quiet, demure, polite, etc. Of course by no means was all of the province like that, but probably the urban elite of Toronto with whom most Americans would have interacted could have been at the root of this idea. Imagine that British stiff upper lip idea with a conservative Protestantism, minus the powerful British Empire identity... And you might get a polite English Canadians.

Again, this question is difficult to answer as (as far as I know) there aren't exactly sources pointing to the cause. So take mine with a grain of salt.

A personal speculation is that perhaps the reason for the perception of the 'polite' Canadian on the international stage is due to Canada's international policies.

Human's love to compare, and being the neighbors of the US could paint Canada in a different light. What I mean is, while the US could be perceived as warmongering, Canada comes off as the gentle giant in the north.

That with domestic policies that offer health care, gun control, etc one can understand where the stereotype comes from.

Truthfully, they aren't any more polite than your average American. The whole idea of them being polite is a widespread joke. I would presume that this joke comes from the fact that Canada is very similar to America but lacks a reputation for warmongering or interfering with other countries.