What kind of issue it is then ? If a regulation permits the doctors associations to set the allowed number of doctors residency, naively it is a regulatory issue.

It's a funding issue. There aren't enough residency slots available given the number of medical school grads. Residency is a requirement to get a medical license--which is issued by the states, not the federal gov. The reason there aren't enough residency slots is because they are heavily subsidized by the federal gov and they put a cap on the funding. No one else wants to foot the bill, so the slots remain limited, thus the licenses remains limited.