A hefty portion of the population hates, fears, or scorns the government. They think putting healthcare (even more) in the hands of the state will either lead to worse results or give the state even more leverage to push them around.

Also, a big and influential portion of the citizenry is served just fine by the current arrangement. If you don't have any long-term medical conditions and you have a job good enough that your employer provides medical insurance, you're doing just fine, thank you.

Finally, there are large and influential groups that are making a lot of money under the current system, and expect they would make less under a more socialized system. Among these are doctors, insurance companies, and pharmaceutical companies.

All these factors add up to a formidable bulwark against large-scale change.