Well, I don't know this for a fact, but I would assume that the United States population is relevant to the debate. We have over 300 million people in this country. On top of that, our pharmaceutical industry and insurance companies are shady as hell. I don't really know if it's as simple as that; if there's anybody here that actually understands the system, then please provide a thorough explanation.
Somehow, I would guess that due to our population density, the waiting lists and times for free surgeries would be significantly longer for US citizens than Canadian citizens.