From Policy Guy at True North:
What we have in single-payer systems … or at least any single-payer system that incorporates enough of the population to have market power … is an attempt to allocate resources without prices. When you don’t use prices, you end up—when government is the purchaser—using politics. And with that comes rationing, political favoritism, misplaced priorities (bridges to nowhere), and a host of other ills.
King adds:
All scarce goods must be allocated somehow. If you won't use price to do so, then it's first-come-first-served, or equal shares, or some other rationing mechanism. Price allocates according to who is willing to sacrifice the most to get it, using a widely-accepted medium of exchange. If willingness to pay doesn't work for you, tell me please what does.
With rationing systems such as queues, good ol' black markets, and bribes, those with the highest willingness to pay often still get the good, whatever the good is. The payment is there. It is just hidden from view.