MORS is fine if you're just dealing with the DoD.
But what about LAPD, or the UKMoD, or NGOs, or any private company?
It's not that a professional society is the be-all and end-all. But it's one of several options on the table that are always discussed and never enacted, for at least 20 years now
Although it wouldn't be duplicative for the non-DOD wargaming community...(national security and non-nat sec policy communities, think tankers, academia etc) It's probably also worth asking if MORS is the right place. My sense is that there has been something of an insurgency of late, where "wargamers" have sought to pry the effort out of the hands of the pure ORists, although I concede that is based on narratives I've heard rather than experienced first hand.
Sure, I agree that there are other agencies and organizations that could use wargaming, but I'm not sure if there are enough of them that are consistently interested in this area to reach critical mass.
My limited experience with MORS agrees with you that they are more oriented towards OR and not towards what we consider to be wargaming.
Again from my DoD centric perspective, we already do a lot of wargaming through modeling and simulation. It's a big, well established industry, and it's tightly controlled, often because they want to control the answers and would not welcome outside help that might tip the apple cart.
Having said that, there are some more independent organizations within DoD. I worked at think tank for almost 20 years and had a bit more freedom while I was there. The service academies might be a good fit for a professional wargaming society, and there are various small organizations within the services doing more scholarly work. I dunno. Maybe it would work.