As someone who likes a different type of games (Fighters), that are often derided for having someone buy the same game over and over, I can tell you why:
Minor changes that most people who don't love the game won't notice. In general, most of us like the game we were already playing, we really do. But that's why we're buying another similar game: it's a new twist on a favorite. Maybe it's the new weapons, or a new feature. In the games I play, it's often new characters, or even balance changes. When you love to play a game this much, you're sometimes willing to pay $60 for the new features as long as they were worth getting, and they didn't mess it up.
Now I can't judge which COD games they messed up, or didn't mess up, because I didn't like the gameplay in the first place, and have always liked the old school arena shooters more (Quake, Unreal, ect).
Also, some people buy the game for the campaign, because lord knows that old school shooters have better multi-player, as do many other shooters, such as Halo, Gears, The new Counterstrike, and even Transformers: War for Cybertron. Some people just want another new campaign, for the same reason that people bought Super Mario Galaxy 2.
PSN:omegamarth. I play fighting games. Washington Redskins 3-6 Baltimore Ravens 7-2