It's not. Car insurance is mandated by the states, which can do pretty much whatever they want. If a state wanted to force its citizens to buy health insurance, they could and it wouldn't unconstitutional. People who didn't like it would pretty much have to either move or start online petitions. The problem is that Congress's authority to mandate health insurance is iffy. They're using their power to regulate interstate commerce to justify it, but whether SCOTUS will buy it is debatable. The ironic thing is, Congress can, under the constitution, completely socialize health care, but it would never actually get passed like that. (We spent most of Monday's lecture talking about this in my constitutional law class, this is what my professor told us about it.)