What if it's just that more people from those schools are actually interested in specializing in those areas? We actually match a lot of people in pathology here at TCOM, and I think a lot of it has to do with the fact that we have a really strong path department and people develop an interest in it early on. There are also likely more DO specialists in the areas with older DO schools because there are proportionately more DOs in those areas in general. It would follow that students there would have more opportunities to rotate in those specialized areas and consequently would choose to apply for those residencies.
I woudn't assume that there are fewer DOs in "competitive" residencies because they didn't match in them. My guess is that fewer people choose to go to DO schools in the first place who have aspirations to become opthalmologists, etc., which is reflected in fewer DOs in those areas.
Historically, what DO schools have done best is train primary care docs. That is changing somewhat, but it's kind of like salads at McDonalds...a good idea, but may take a while to catch on...