What do you think it is?
I think most URMs with average to above average DO school stats (3.4+, 26-30 MCAT) could get into state MD schools. Honestly, I wonder how many URMs with significantly lower stats are really applying to any medical school. I wouldn't have thought I had a chance at a US school until I found this website, found people in a similar position as me, and ultimately applied and got interviews, and I'm from a medical family.
Also, to be completely honest a lot of the DO schools are not in particularly diverse areas. Granted that's probably changing with the explosion of DO schools in the last 15 years, and hopefully we will see some changes in graduating classes.
In general though, as you alluded to, DOs aren't really advertised a lot, especially in areas that don't have a lot of practicing DOs. I know that a lot of schools are starting PRIME programs and specifically recruiting members of some minority groups, but again that's not something that we will see have a significant effect for a few years now.
I think a lot of what makes people become doctors is knowing someone who's made it, a mentor of some kind. A lot of times a successful URM would be in a particularly good position to advertise for the DO degree. Ultimately though, I think most successful DOs simply advocate medicine in general as opposed to the DO degree itself. I may be wrong, its not like I'm surrounded by DOs or anything.