Federal law has set a minimum for car insurance, I think, might be per state (I think you have to have a minimum of liability). Many states have requirements that exceed the Federal min. They don't contradict it, but require extra, which so far has been held to be fine.

I don't know the full history about why you cant buy health insurance across state lines, but I think it's a blend of different state's regulations and not wanting to be beholden to interstate commerce clause regulations (ironically, the provision that was used to justify the ACA mandatory insured or penalty as a tax ruling.)

I never have nor will understood how the interstate commerce clause was used to justify the ACA "tax". The mandate is effectively compelling intrastate commerce (I know of know interstate medical insurance available) under the guise of regulation interstate commerce. This seems to fly against the letter and intent of the constitution in regards to interstate commerce.