The game came out a long time ago and compared to a multiplayer game with millions of buyers like Call of Duty it can't keep its playerbase. If it were multiplayer with constant content updates people would stick around a lot longer.

Me and this other guy was talking years ago about an adult free to play multiplayer game like Game of War where you own brothels and whack off other players, kill their babes, infect them with disease, upgrade your own defenses, hire pimps, bouncers, etc. There's definitely more thought to be put into it, but that's the basic premise and as long as it's not p2w with everything except cosmetics able to be purchase with currency earned in-game, it should be great.