So, then you start residency training, during which you often work 80+ hours per week (used to be worse) and make a starting salary in the $35,000 - $45,000 range, and your loans are coming due. The shortest residencies are 3 years, and many go on for five years. Then, if you sub-specialize, you do a fellowship, also for relatively low pay for an additional three to seven years. After that you can go out and start practicing.
If you go private you will be the low man/woman on the totem pole and thus make significantly less than others in the group, and often be saddled with extra holiday and weekend call. The bottom line is that it's not going to make sense to people to go through that if at the end of the day the government decides what you can earn, and if you can earn as much or more doing other things that require less years of training and time commitment.
The ignorance of our community organizer and chief in this, and most matters is breathtaking. In one breath he's telling us that Wall St. will play a less dominant role in America's life and that traders should now become engineers and computer programmers while in the next breath he's pontificating the value of universal health care. Where are these additional doctors and nurses going to come from? Thin air? And as you've stated, where is the incentive to become a doctor, with all of the expense in time and treasure involved, only to be told what you can make and what patients you'll be required to administer to? These questions do not have easy answers as he will soon find out.