The abridged version is that they were working on it but the head of Kaiser convinced Nixon that private health insurance was a better option. Fast forward a few decades and there is too much at stake for the companies providing the care^insurance for a graceful change. Large companies also don't give a hoot because they're able to offer corporate self-insured plans which constitutes a competitive advantage against smaller employers.

My personal opinion is that the ACA was also complete garbage. Either go single payer (or at least an option to buy into a single payer) or don't go at all. The half assed approach we've ended up with is the worst combination as the individual is forced to purchase insurance in a market with perverse incentives on all sides and no real competition.

There are lots of other systems besides single-payer (like the NHS grandparent mentions liking) or the US. It isn't some law of nature that anything "in-between" is doomed to failure.

For example, read a bit about the Swiss system:

https://en.wikipedia.org/wiki/Healthcare_in_Switzerland

The ACA isn't all bad. One thing it did do was open the market to more providers. Prior to that every state had their own insurance companies (ie. BCBS-FL, BCBS-VA, BCBS-WA). And if you wanted it there were maybe 2-3 companies you could choose from.

A hefty portion of the population hates, fears, or scorns the government. They think putting healthcare (even more) in the hands of the state will either lead to worse results or give the state even more leverage to push them around.

Also, a big and influential portion of the citizenry is served just fine by the current arrangement. If you don't have any long-term medical conditions and you have a job good enough that your employer provides medical insurance, you're doing just fine, thank you.

Finally, there are large and influential groups that are making a lot of money under the current system, and expect they would make less under a more socialized system. Among these are doctors, insurance companies, and pharmaceutical companies.

All these factors add up to a formidable bulwark against large-scale change.

The direct answer to your question is that it's a complicated historical thing, where American doctors in the 1950's wanted to retain more control over their practices and happened to have the political power to make that happen.

We Americans have a pathological fear of socialism and since the 1980s it's been ingrained in the collective psyche of the nation that government is bad. You'll hear people talk about how government can't do anything right but we have the greatest military in the world. Such contradictions are endemic to the right leaning people of the country. Left leaning people of the country have a differing set of contradictions.

There is also a coupled fear of a poor person getting something they don't deserve. This has foundations in religion and in the development of the nation. There was a time (over a hundred years ago) when a man(family) could do it on their own and live off the land. This inculcated in us the idea that being poor is associated with being a slouch. Such a time has long since passed but that ethos is still in the collective consciousness.

There is also a pathological fear of taxes. It is widely believed that government is always more inefficient than a privatized solution. We had a Vice-Presidential candidate laugh at the notion that paying taxes is patriot and this while her son was in the military during a time of war! A lot of people think of themselves as some sort of Daniel Boone character that can do it on their own if only government got out of the way.

We do have a national healthcare system for elderly, members of the military, and most children. Everyone else can fend for themselves.