If one does not get matched and one is a citizen (or a green card holder), s/he really must have a lower than average USMLE score and/or GPA in med school. Or s/he made a mistake in shooting for too high of a program (e.g., trying to get into UPenn/JHU program when one's score isn't that good).

Foreign medical graduates who scores above 65-70th percentile have no problem getting into residency programs (I've made a comment above with more details as to how I know all this). For a green card holder, you just need to get ~60th percentile of the USMLE score (which translates to about 240+ in step 2 CK and step 3 USMLE scores).

All I wanted to say is that the bar isn't too high for a US citizen to get into a residency. That doesn't mean though that the path to get into residency is easy. It is extremely convoluted and requires a lot of persistence and planning. That's where (along the way) people fall through the cracks.

The application process is highly convoluted and not very well designed. Some people just fall through the cracks.

But yes, also a small number of people just don't have the intellectual ability to handle it, although schools do a lot of screening to minimize this. Graduation rates at schools are typically 100%, with maybe 1 person every other year failing out.