I think it's because we have a lot of alternatives now, for different tastes. Back in the old days of Goldeneye and Halo, those two were pretty much the only really good multiplayer shooters. Halo 2 dominated the console online space back then because it was the only real shooter on the console which was the only proper online system at the time. Now we have three systems each with online capabilities, and a ton of online games spread across them. As there's no longer a select few games for everyone to flock towards to, gamers are scattered across the broad spectrum of multiplayer games. Even the most popular games like Call of Duty 4 has to fight for the attention of your friends looking for games to play online with.