Very unlikely that's its disallowed.
For me personally, it's a matter of comfort (or discomfort) with my ability to effectively treat certain body regions. I'd rather not be advertised as someone that can do OMT but pull it out of my toolbox at my leisure for select patients and complaints. I don't like being booked for exclusive OMT visits, as then it's expected that I treat potentially any body region.
First those who don't do it at all, it makes sense. Questionable efficacy combined with needing to do it fairly regularly to keep your skills up means it is just easy to take a backseat to everything else. Primary care is difficult, largely because of the breadth required. I need to be able to do a fairly long list of office procedures, but I don't need to do OMT.