Couch co-op has never been an assured feature with games, especially recently. Since 2000, developers on the PC platform have clearly ignored the co-op mode in favor of dedicated multiplayer modes with big name first person shooters like Half-Life 2, Doom 3 and Quake 4 skipping on the mode. Games with teamplay orientated multiplayer modes like Counter-Strike and Call of Duty have picked up the fundamentals of co-op based games and thanks to their easy accessibility over the internet, have become wildly popular as a result (especially in comparison to the limit of 2-4 players in a usual co-op game).
There will always be developers that include couch co-op modes in their games (Bungie, Blizzard and Ubisoft come to mind), but there's unlikely to be a massive resurgence in the number of couch co-op games mainly due to an internet based takeover of team based gameplay.
[Thanks, Daniel Zuccarelli]