Because Modern Warfare isn't modern enough any more.
The next Call of Duty game is called Call of Duty: Advanced Warfare. Here's the game's first trailer.
If you thought the second Black Ops game took some technical liberties, hoo boy. This one is going full-blown science-fiction. Note that the game doesn't just star the vocal talents of Kevin Spacey; the guy is literally in the game.
The trailer has a release date pegged for November 4.