A recent WSJ article explains why:

Socialized health-care systems fall short in these critical cases because governments strictly ration care in order to reduce the explosive growth of health spending. As a result, patients have less access to specialists, diagnostic equipment and pharmaceuticals. Economist David Henderson, who grew up in Canada, once remarked that it has the best health-care system in the world – if you have only a cold and you’re willing to wait in your family doctor’s office for three hours. But some patients have more than a simple cold – —and the long waits they must endure before they get access to various diagnostic tests and medical procedures have been documented for years. Montreal businessman George Zeliotis, for example, faced a year-long wait for a hip replacement. He sued and, as the co-plaintiff in a recent, landmark case, got the Supreme Court of Canada to strike down two major Quebec laws that banned private health insurance.

(via Repairing the Healthcare System and Evidence in Motion)