Thats the thing, that if you go through AOA match, and match, you have to withdraw from the NRMP or allopathic match. If you are confident that you can obtain allo spot without going through the AOA match, fine, don't participate in it. However if you are taking a risk applying for the MD spots, but are not sure, and maybe quite a qualified candidate for a DO residency, you lose that AOA spot, if you bypass the AOA match. Combined match would lallow the people who want to apply for MD spots but want to have a safety net, without losing a shot at DO residency and at the same time not having to settle.
There have been articles in the DO magazines, and the reason for it that they feel that those who end up going for mediocre residencies get a chance to try to get something better without nervousness. That means that those mediocre residencies, and i am not saying ALL Do residencies are mediocre, but some truly are, might not get enough spots. So that's the main reason that you probably won't see combined match.