Despite how big a problem healthcare is in the US, most voters fear any change that may put their current health coverage at risk. Most people under 65 have (or feel like they have) decent-to-good health insurance through an employer-sponsored plan and don't necessarily see a big benefit to throwing that away. People over 65 are fiercely protective of their Medicare benefits and tend to worry that Medicare for All is going to make their healthcare worse. (Judging by the amount of "shocked Pikachu face" we see lately from Trump voters who expected him to fix the economy, there's a critical group of swing voters who seem unwilling or unable to accurately judge candidates based on their policies vs. their vibes.)
I don't agree with those people. I'd love to see our healthcare system dramatically overhauled. But between the polarization and risk aversion of American voters, "a quality National Health Service" has not proven to be a winning platform so far and is unlikely to be in the future.