The schools are limited by their physical capacity. There's just so many students that can fit into lecture halls, student microbiology laboratories, etc. Once they are maxxed out, you have to build new schools.
It doesn't make economic sense for schools to want to restrict their enrollments.
"managed care" is making medicine a less lucrative and otherwise less attractive field, along with increasing medical liability and health care entitlement that forces doctors to care for uncooperative and unappreciative patients. These are the factors that discourage young Americans from pursuing medical careers.
University and other education settings are not competitive economic environments. They are run more like guilds or non profits. Grant, government funding, mission and tax breaks put them in a different economic catagory. The idea is to extend influence but not necessarily put out more product.
External factors such as market forces have changed the setting...agreed. But people go into medicine for many reasons, one being economic. What MDs are doing is trying to reposition a few on the top of a health provider ladder that they hope they can control.
Most socialized systems are stuck with paying for care that politicians don't WANT to pay for as it takes monies frpm other "projects"...so they are stuck with giving lip service to more access and benefits, while at the same time erecting subtle barriers to access.
http://www.outsidethebeltway.com/archives/2005/02/controlling_health_care_costs_by_controlling_access/