Because you can expect less than that number of people to play the online version.
I don't know if this comment is universally true. Perhaps it is for this game (I wouldn't really know) but take a look at the FPS's for GameCube as opposed to those for XBOX or PS2. Even though "Geist" had fantastic gameplay and split-screen features, it was far less popular than the Halos because of GameCube's lack of online features.
I think more people will be inclined to play a game where they can interact with other people, regardless of the gameplay. But, on the same token, if the gameplay is really that bad the opposite could be true.