You’ve got the answer right there. It should be nationalized. And a major reason it will never be nationalized is because of lobbying by health insurance corporations.
I’d maybe vote against state level single payer, let alone taking over healthcare entirely, because I think it’d be a ton less efficient than doing it at the federal level—enormous amounts of money already go to various federal healthcare programs and it’d be better to pool that, and also lack of state level control over their markets is likely to cause problems. I dunno, it might depend on the law, but my gut reaction is a “no” vote on that, too.
Anyway, as for the other question, I believe that the rise of the postwar think-tank industry (a mash up of k-street and Madison Avenue, but for white papers, as it quickly became) and deliberate party and industry efforts to shift public sentiment in pro-market directions, tied up with lots of government spending on anti-Soviet messaging (were they bad? Oh, yeah. Were all the things the government tried to lump in with them and decry as un-American bad? Ehhhh… I mean atheism is among those, see the change to the pledge and to our money at the time, among other things, and I think atheism’s fine) is probably why healthcare can be this incredibly messed-up and nationalization remains not just not-popular-enough-to-pass, but entirely outside the Overton window, yes.
M4A is nationalized health insurance if not quite nationalized healthcare, but the point remains that the only reason we don’t have major steps in this direction is because of people in power who will not let it happen.