The insurmountable issue with single-payer is that the government is directly involved with authorizing all of the care, and therefore is in direct control.
Pavlov cannot be blamed for the fact that government is not trustworthy to hold the keys to health care.
I don't know that I'd trust insurance companies any more with the decision over whether they'll approve a treatment plan that could save my life. At least the government has to pay attention to the will of the people to some extent. The insurance companies care more about the will of their shareholders.