Yeah that's a hard question to answer, and it's one of the dumber questions they ask on secondaries, in my opinion. It basically forces you to come up with a load of bullcrap, because most people apply to one or two top choices, and the rest are just backups in case they dont get into the the former.
Location is a good one, though. Check to see if they have problem based learning. If you can somehow contact people from the school you could also throw in that you heard great things about the school from students. Maybe you're attracted to how much experience the school has (this is good for KCOM). Do they have a good reputation? Ask any DO's you're shadowing if they know anything out of the ordinary about that school. Call the school and request information about how well students do on the boards. If they do well, mention that as another reason you want to attend their school.
Is the med school on the campus of the main unversity or is it located elsewhere? For example, would you rather be in a school like Nova that has the medical school on the campus and the med students interact with the other allied heath students (dental, optometry, physical therapy etc), or like OSU and I think UMDNJ where the med school campus is a building located away from the school and the only people you see are other med students. There are pros and cons to both and you could discuss why you would like to attend a school that does whichever you prefer.
Do you want to be in a school with a big or small class size? Osu and UMDNJ have between 85 and 90 students, Nova has 180ish and I think New York has around 250 or so. Mention how the size of the class appeals to you for one reason or another.
For OSU, I mentioned the problem based learning system they have, the fact they they are nationally ranked in three categories that define osteopathic medicine (family practice, rural medicine, and primary care), as well as the smaller class size. It's hard to argue that location was a reason seeing as I'm from Florida, but I was looking to leave Florida and experience another part of the country so going to OSU gave me that opportunity, which I think I noted.