You underestimate the scale of the challenge.
1) Don't kid yourself, single payer is better for under/uninsured people but there will be plenty of folks with plans who will see quality of service decline. It takes months to get an MRI in Ontario, you can get one in 24 hours just a few hours away in Buffalo. There is a sizeable number of people who will prefer the old system and they are the most powerful people.
2) Single payer is cheaper but it was also implemented in those places decades ago. Price levels in the US will need to be cut for services, drugs, etc. This isn't "efficiencies", this is straight cutting what insurance will pay service providers (and banning any other payment than that insurance). The doctors, hospitals, diagnostic firms, pharma will be screaming bloody murder and threatening the apocalypse if you try to bring US pricing down to global benchmark levels. If you maintain current pricing and then extend quality coverage to tens of millions of new people you will blow the budget.
This isn't passing a piece of paper and then forgetting about it. It's the transformation of 1/5 of the economy, going against some incredibly powerful special interests, with a lot of resistance to change. The chance to screw up is enormous, and the level of care and attention that would be needed to implement cannot be understated.