it's prob just a matter of having a more level playing ground for medicine.
for dental schools..
a. most dental students become general dentists..so while you can rank schools by how many students are selected for specialties..it wouldn't be a fair comparison. plus, even if you were to compare general dentist training...does it really matter if 1 student at X university does 100 amalgams vs. another student at Y university that does 300? yeah, one looks better than the other on paper but it comes down to the skill of the student as well. dentistry has more of a "hand skill" requirement.
b. dental schools differ depending on research and clinical philosophies
c. residencies are optional
d. research isn't as comprehensive
vs. medical schools where..
a. there is a larger variety of specialties to choose from, and there are more seats for those specialties...comparison is on a more level ground
b. all medical schools have a fair mix between research and clinical aspects...particularly MD schools...not so much for DO schools. finding a DO school that is solid in both clinical and research aspects isn't common, as the main DO philosophy caters to underserved areas (which is why you find mostly family prac doctors that are DOs)
c. research is much more comprehensive
edit:i also think politics plays a big role as well. physicians roll deep, to be frank...more so than dentists.