I think you're comparing two different genres. CoD might have a huge following, but it's an FPS. We are dealing with an RTS (and a rather long one at that), many people don't have time to put hours+ into games, whereas FPS you can go in and out rather quickly.
On Topic: I think several factors come into play here, not only the time issue, but single-player is so much different than multiplayer that some (like myself included...though I play comp stomps) don't want to try because they believe they will get annihilated in the first hour of the game.
Hope this helps, Airship_Artillery
