The people essentially have been demanding it. As several others have mentioned in this thread, 70-80% of the public supports universal healthcare in some form. For the nitty-gritty, basically 50 years ago and earlier, extremely wealthy people realised that their preferred policies weren't especially popular with the general public. They identified that their main issue was that what they had was money, not people. So they embarked on a decades long quest to give money the same (or greater) political power as individuals, culminating in the Citizen's United Supreme Court case. I bring this up because it essentially means that the People demanding it doesn't matter, because while there may be a couple hundred million people asking for it, there's a couple hundred billion dollars asking to never do it. It's gotten so bad that there's a kind of perverse, Stockholm Syndrome effect starting to happen to. In 2016 there was a big dust-up during the Democratic Primary where the Culinary Union in Las Vegas/Nevada didn't want to endorse Bernie Sanders, the most pro-Union candidate in decades, essentially because medicare-for-all would remove health insurance as a bargaining chip