Randy Barnett from The Washington Post said it pretty well:
Regulating the auto industry or paying "cash for clunkers" is one thing; making everyone buy a Chevy is quite another. Even during World War II, the federal government did not mandate that individual citizens purchase war bonds.So do you want the federal government to have that kind of power?
If you choose to drive a car, then maybe you can be made to buy insurance against the possibility of inflicting harm on others. But making you buy insurance merely because you are alive is a claim of power from which many Americans instinctively shrink.