One of the reasons its free, or at least cheap (elective and semi-elective surgeries are fee-for-service in Canada) is that Canada and much of the world gets much of its technology and research findings from American universities and research institutes. America spends the money, and the rest of the world benefits from it, even expects education in it.
Another reason is that service is limited, often severely. Fewer facilities, only in big cities, have modern up to date technology found in almost all facilities in America. Maybe that is one of the reasons why so many Canadian doctors and health care workers are educated in America?