First, I think it has to do with financing. Some schools are private and have very high tuition costs which international students would have to cover upfront at the beginning of each year for a total of 4 years. So the school is trying to prevent int. students from dropping out because of money issue. So unless you have all 4 years tuition + other expenses ready to go in an escrow account or other reliable source, some schools will not be convinced to accept international students. Also some schools receive federal/private funds based on the number of U.S. citizens they enroll so they would not want to pass on those either when the pool of qualified international applicants is not very high anyway.
Also a big issue is that by accepting international students in their medical program, the school will have to setup a very dedicated/experienced staff who can deal with the requirements from Homeland Security. So not all schools want to go through all this headache. 😎