Joe Flower has a good post explaining why free market competition fails in health care. Go read the whole thing, but the short version:
Medical demand is wildly random but tends to be absolute. There is little way to predict who, specifically, will need most kinds of medical care. But, when you do need it, you have to have it or you’ll die.
Medicine is usually beyond the layman’s understanding. Consequently, the buyer has to rely on the doctor’s (who is also the seller) representation of what the buyer needs. “So the seller is agent for the buyer, the seller is rewarded for doing more and punished for doing less, and neither the buyer nor the seller can easily tell the difference between what is really necessary and what is optional.”