1. The political power of providers in the USA. Groups like the AMA (the AMA is in effect the doctors union). They often assert this power through over regulation and excessive licensing which insure that they will be used to do things that less licensed people could do. They also use licensing to keep supply low and in turn prices high.
2. Monopsony enabling the squeezing of providers is a big factor in how other countries achieve lower healthcare spending. Monopsony is a monopoly of buyers. So for example in Japan the government pays for almost all health so they can tell the providers (doctors and hospitals) what they will pay. The only alternative to accepting the low compensation that the providers have is to leave the business of healthcare. This gives the single payer great power to reduce prices but creates the shortages and waiting that we hear about.