As a social organism, the multiplayer game (seems to be the equivalent of the word "game") nowadays is very different from its older brother, the single-player game. While it is usually deeper - in that you are usually sparring against human minds and therefore make complex psychological decisions instead of just solidifying dexterity for execution, as is in the case of most singleplayer games that require skill of some sort - it is also inherently shorter-lived.
The reason? Plain and simple - when the people move on to a new game, the scene dies, and there is no reason to play the game anymore.
Maybe this is why Mario Bros. survives to this day - you can still pull out a NES or emulate it and relive the childhood glory of an Italian plumber hero, wistful of the speeding technology of the 21st century, while Street Fighter II doesn't. Though both, I can argue, are equally important to their genre, and to the entire family of the video game.
Maybe this is why you can still find a shrine to, say, Shining Force, but not one for Madden 2004.
There's something sad about this - to trade for depth, socialness, and growth, the multiplayer game sacrifices lifespan. Its goal is not to be remembered forever, but to live as a bright spark in one's life. 30 years from now, only the diehard game historians will remember Soul Calibur, but Mario will still live through reincarnations and historical impact.
You can also think of the multiplayer as a parasite - blessed with quick energy and growth through the social network it feasts on, sapping the competitiveness and ingeuity of the real game players of our generation, then quickly receding after its host becomes disinterested. With the death of competition comes the death of the game.
Sadly romantic either way, whether you look at it in a positive or negative light.
P.S. guardimpact.com was the primary source of incredible high-level strategy for the original Soul Calibur. Alas, with the rise of SC2 (even that is on the decline, competing with just better-sold games such as T5 and DOA series, soon to be replaced by SC3), another great game bites the dust.