Honestly I think it has a lot to do with the popularity of Fox News and its bias against any centrist/left policy. Also, keep in mind that the US was for decades fighting a war against communism which branded any socialist idea as evil and even going so far as to overthrow socialist governments. So, there are decades of propaganda that needs to be overcome.
In fact I think we have the millennials to thank as they grew up after the cold war and did not experience much of the propaganda that older generations did, so they are more open to policies like universal healthcare.
edit: quickly downvoted? If you disagree that's fine, but at least share what you think I've got wrong.