I also think it has to do with what the most common treatment plans are for a given disorder; PDs, like Borderline, usually want a set "course" of therapy for X weeks, etc. and are more along the lines of a treatment for a physical disorder. The insurance companies like the known, but kind of have to put up with the "serious" too, like major depression, etc. But the lesser-known illnesses where there's no agreed-upon treatment or time-limited treatment, those are too squishy for their liking.