There's probably a combination of reasons.
1) Most (read: all but a handful) of DO schools are in rural areas or small cities, so to get adequate training for hundreds of students, they have to branch out to other regions.
2) The DO schools in densely populated cities are usually very close to multiple MD schools that were either already around or own a good chunk of the hospitals in the city. They are already mostly filled with MD students rotating. Many DO and MD students rotate at the same locations, but if the hospital is owned by an MD school, there will definitely be more spots for the MD students than for the DO students.
3) Many DO schools have a tradition of "preceptorship", primary care, and rural medicine, so you have some rotations focused in outpatient primary care. We're talking small doctors offices. You can't send 20 kids at one time to a small outpatient office (or at least I hope you can't) run by a few docs. Because of that, you will be sending people out further and further away. For whatever reason (cost, tradition, whatever), DO schools cling pretty tightly to this model.
4) Most DO schools, unlike most MD schools, don't own their own teaching hospital. With most MD schools (not all mind you) there is a teaching hospital nearby that provides most of the clinical training to the school's students. This is a combination of COCA's regulations on ClinEd being more lax than the LCME's and probably some of the things above (rural location of schools, etc.).
I'm sure there are more reasons, and others can point them out.
Again, like I said, you can get a good clinical education at pretty much any DO school (especially those mentioned in the title), but there will definitely be some variability, so you may have to be more proactive with your learning than you might like.
.
(slight exaggeration, but the metro area is really huge...)
