This never made sense to me from the get-go, but I guess sometimes you try something different just to see if it would be better, and you find out before long that it's not. The explanation when they first did it was that Joint Chiefs would be included on an as-needed basis, but that based on the nature of the NSC's day-to-day work, they didn't need to be there all the time. Bannon did, I guess, because the politics of security decisions needed to be consistent with the substance.
Oh well . . .