Bones lays out the situation for OMM-only practicioners pretty well. It is an unfortunate situation because I feel that OMM should not be restricted to the "rich," but with today's insurance situation this is the reality.
Let me add, though, that some PPOs do pay for OMM, provided you know how to code for what you do, and Medicare pays pretty well too. Medicare will pay about $95 for a visit, provided you treat 5-6 body areas (something which is easy to do in a visit,) and about $135 for an initial visit. I believe PPOs pay similar rates. Unfortunately, considering you would probably see 10-15 patients a day, the cost of hiring a biller to process your claims is too high. Thus most practicioners will simply provide a Superbill so that those patients whose insurances will pay can get reimbursed.
The upshot of not making insurance contracts, especially Medicare contracts, is that the practicioner is free to charge patients whatever they feel is appropriate, i.e. reduced rates for patients who can't pay the whole fee. This would be considered fraud if they charged insurances a different rate.
Finally, with OMM specialists making such good money, many of them can afford to provide free or reduced cost care at community clinics, something which is difficult for your average practicioner to afford.