Competitive pressures, contractual obligations, the fact that they can be sued, and the need to keep their customer bases at least relatively happy to prevent them or their employers from switching to another company. Government has no such constraints in any of these regards. It can do whatever it wants whenever it wants and whoever doesn’t like it can go pound sand.
Look at what goes on with Social Security and Medicare. Their recipients have no idea from one year to the next what is going to be done with their benefits. They might be cut, they might increased, some aspects might be eliminated while others are made to require greater co-pays and deductibles. Their fate and much of their day-to-day living expenses and quality of lifestyle are completely at the mercy of the machinations of Congress and the administrators of those programs.
And so it will go with single-payer health care. Coverages and qualifications and limits will rise and fall with the vicissitudes of governmental economic and political considerations and everyone will be at the mercy of whatever the government decides it will do whenever and however it decides it will do it.
And people will have little practical recourse. Yes, private care options may still be available but how many people will still be able to pay for private coverage or treatment on top the money the government dings them for in order to pay for its program, and how many private treatment centers and physicians will remain in business once most of the populace is driven into the government program? My guess is not many, in either regard.