It is more efficient for a Dr to have a suitably qualified Nurse that can do it and not have a Dr supervise every injection, that being one that has completed an immunization course about 18hrs of study. A Nurse that has not completed it needs a Dr to supervise each injection therefore not efficient they may as well do it themself. Remember the Dr still has their regular patients that are seeing them for other more serious illness.
The vaccination program with a qualified Nurse is far more profitable for the Dr. A surgery that can do as little as 150 per week can be excess of $15,000 over 6months just for doing them.