Why was the Doctor

Since it's not in the show, this would all be conjecture, but I would think it would naturally occur to the guards who are disposing of the bodies that they could make some side cash with black market organs.

From there, it was just a matter of finding a contestant that was a doctor and recruit him to remove the organs in exchange for game tips. If the doctor is participating in these games, then there would be no moral issues to contend with.