Because drugs and medical professionals don't grow on trees, citizens of modern democracies must choose where they want to be between two extremes:
1. Where healthcare is nationalized, but to avoid bankrupting the government, you have to limit what care is available to people.
2. Where healthcare is completely private, and instead people rely on private insurance or savings to pay for treatment.
The irony here is that proponents of #1 argue that healthcare is a basic right. And it's true that in a private healthcare system, some people will not be able to get treatment. But it's very hard to see how a policy denying people available treatments - which a nationalized program invariably does - is an improvement over allowing people who can afford medicine or coverage to purchase it.
Nationalized systems also raise the question of whether I have a right to expect you to pay for my lung cancer treatments after 40 years of smoking, or my physical therapy after a bad rock-climbing fall. By insisting on paying for my healthcare, you're now placing limits on my lifestyle - and I'm for damn sure going to place limits on yours. Some systems (for example Sweden) do try to take some of these into account (for example, smokers pay a higher tax) but you can't take into account every example of willfully unhealthy behavior, and we all get stuck paying for the medically irresponsible. For example, in the Swedish system, to my knowledge alcohol intake, rock-climbing, or even simultaneous participation in both does not figure into the scheme.
At this economic low point in late 2008, the less wealthy countries will be tempted to turn economically to the left, especially on questions like this, and it behooves the world's free marketeers to stand especially firm against the tendency now.