Because one Call of Duty game per year just isn't enough.
The "remaster" phenomenon has been quite a thing with this generation, where developers will take a title from the last generation, slap on a few more graphical bells and whistles, and then sell it on their newest system for full price. Activision Publishing CEO Eric Hirshberg thinks remastering games is a great idea, and is actually thinking about it for its Call of Duty franchise.
"If done well, I think [remasters] can be great," he said. "You talk about nostalgia, and people have such connection to the games they love from the last cycle. They want to see what it would look like if someone did it right for this cycle. It's always the opportunity cost, meaning we need every body we can get to make the content we're already committed to for our new games. It's always a matter of finding great people to do that work. I would love to play Modern Warfare 1 or the original Black Ops. There's certainly a deep well there. No announcements, but it's something we talk about and think about a lot."
On the one hand, this can be seen as milking a franchise that is already being exploited as much as possible, on the other hand, I certainly would not object to a HD remake of Call of Duty 4.
Source: Game Informer