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.