One of the biggest factors is the US government pays the salaries for doctors in their residency programs. They indirectly set, and therefore limit, the number of seats available in US programs by the amount of funding they provide.

There are many other failures as well. For example, most programs requiring a bachelor degree to start. This artificially raises the cost. There are a few programs now that intake high-school graduates and add a year or so to cover the basic courses that they need, such as anatomy and physiology, but effectively cut off about 3 years of higher education.

I am curious what the post above was alluding to.