We already do have universal health care for the most expensive groups to insure (lower income households and the elderly), and technically have it for everyone

We already do have universal health care for the most expensive groups to insure (lower income households and the elderly), and technically have it for everyone in that hospitals aren't allowed to deny life saving care to anyone regardless of their ability to pay (which is expensive, short sighted, and quite inadequate overall). Adding the rest of the population to the existing public insurance system would not cost much financially, but it would be a political catastrophe for whatever party implemented it if it didn't go well. In short, I don't think anyone seriously argues the US can't afford universal health care, but the real and perceived risk of change is seen as too great politically.