This is a really nuanced question:
Philosophically Americans want choices, the freedom to choose physicians and procedures. Financially this is not always an option for the majority, but the idea of limits to freedom of choice has made single payer "contrary to freedom." The idea that government chooses which services are available is offensive to enough people that Single payer gets dismissed outright.
Managed care or accountable care organizations that would support a single payer option by design would have to limit choices. For example, not all physicians would opt to be part of a single payer system, and there are enough people with money that our very best medical professionals may not even be part of that system. Why?
If you look at medicaid and medicare payer rates for full encounters or drgs (a package of services associated with a diagnosis), you'll see providers don't always make money. Frankly sometimes a physician/practice loses money in our current system.
Prescription costs and durable medical equipment/devices and medical technology costs have far outpaced inflation.
For profit hospitals.
Unfunded EMTALA.
The failure to pay Risk Corridor under the ACA; and the chilling effect this had on the insurance industry participating in future government health plans.
(and dozen other topics that could fill hundreds of pages of text)