Well they don't own them yet obviously, so they can't just put them on for that reason. Once they do you'll likely see all of the older ones on there.
After 2 years most people are no longer playing the multiplayer, buying battle packs, buying skins, etc. So Microsoft would have no reason currently/previously to pay Activision for a game on game pass that isn't going to have a bunch of additional in game sales.
Hold on...if people are no longer playing the multiplayer, buying MTX etc., then why would MS even bother putting the old CODs into GamePass? If that's you're thinking to why they haven't made an attempt to get older CODs in there so far, for reasons completely aside the single-player campaigns, then why does that suddenly change once they own ABK?
My reason for even suggesting they try getting an older COD release in there, if they were able to do so, is because of the single-player campaigns. Because to me, I'm still under the impression GamePass is useful for games other than just the live-service side of a game's content, or only live-service GaaS-type games. But if it's not, then there is practically no reason for MS to add older CODs to GamePass if they've been unable to add them beforehand.