few reasons. my specialty of choice has many midwest locations i can apply to and be competitive for, but the only midwest programs in the DO match are 500 miles away from my home. in addition, many acgme programs are associated with academic hospitals. there are very few DO academic based programs. there is also a little more prestige with the MD match. even though the training could be the same, University of X sounds better than Bunkydrop Community Hospital. Another program is that many of the programs in the DO world are small. Very small. In my specialty, the average program size in the DO match is 4-5 residents. In MD, you can get anywhere from 5 to 55 residents. So if you are looking for a huge amount of pathology, 30 residents per class may be desirable. Also, if you don't like OMM, the MD match lets you forget OMM after level 3. there can also be a ton of personal reasons.
but as stated above, this will all be irrelevant in a couple years.