Occupational medicine has a few facets. Many PM&R docs work with occupational medicine clinics dealing with the work comp/disability medicine/fitness for duty side of things. But those jobs that are primarily worksite or environmental medicine are better suited for the fellowship trained OccMed doc. In addition, most PM&R docs would get bored doing all Occ Med. SO much of it is DOT physicals and pre employment testing that it can be kinda boring.
I do a fair amount of Occ Med in my practice, but it is mostly injured workers and disability/impairment ratings.