The fundamental problem is US citizens are accustomed to getting the health care they get on the timing they get it. These single payer systems have never had immediate access to cutting edge care, essentially on demand, that Americans do. Sure, Europeans laugh at it, until an American needs a procedure and can have it done in three weeks.
You are never going to be able to provide the access to the amount of care that Americans are accustomed to, on a single payer system. It's a fundamental impossibility. And the vast majority of Americans actually rate their insurance very highly.
I'm not saying one is better than the other even...there is a strong case to be made that Americans use way too much health care and prescriptions, get unnecessary tests, go to the doctor too much. All of that is probably true. But good luck being the party to tell people they have to give up what they've always had.
It takes a matter of weeks to get a vasectomy here. In parts of the UK, it can be nine months for the assessment, and seven more months for the procedure. It cost me $2k, I assume it's free for them. It doesn't matter if that's morally right to wait a year plus for a common procedure, if its "no big deal" with some planning, or whether British people seems to be perfectly ok with it. Americans will never accept going from the first scenario to the second scenario. Like 80% of Americans have fast access to virtually all the health care they need or want, and are not going to give that up, even if it is for the greater good, makes more sense in the long run, etc.
You can't take away something people are accustomed to having, even for their own good, on the basis that other groups that never had it don't miss what they never had.
We need to have a better system, but one that doesn't provide the majority of the people with the access to the majority of care that they get now, at high rates of satisfaction, is a non-starter. Which wipes out most of the Canada/UK style schemes.