Keep in mind that for some Americans, there health care IS the best in the world.
Now, I'm not defending the American system as a whole... it definitely needs major reforms.
But, lets say you're a middle-class white collar worker in the suburbs. Your health care is covered by your employer, and your job is pretty stable. You have no problem getting treatment when you want it, waiting lists are relatively short, and because its payed for by your employer, you never worry about cost.
Along comes Sanders, and says "I'm going to take that away from you and replace it with something else". A system that is known to cause delays and waiting lists in the other major country that has tried it. Even if its better for the country as a whole, your personal circumstances might differ. So even if that voter didn't like Trump and were thinking of voting against him in 2020, a certain amount of self-preservation may kick in.