In basic economics, when standards are raised, less people become fit for a certain job. In this case, the AMA is restricting the supply of doctors by making it tougher overall to get employed as a doctor. They have made college longer, more rigorous, and more expensive to get a medical degree which disincentivizes people to desire to be a doctor. However, with higher medical standards, only the best of the best doctors will get employment.
While we have better doctors now, it is harder for people with lower incomes to get to them because they are expensive and few and far between.