I know hardcore co-op fans hate to hear it, but co-op is VERY difficult to implement into any story-driven FPS. It was a snap with Doom, since it was just wave after wave of enemies, but to tell a story with more than one person present is just a huge commitment, and statistically most gamers don't play co-op, which is why more games don't have it.
There are exceptions - System Shock 2 being a recent example, but don't hold your breath for most games. Developers just don't see it being all that popular. Whether this is due to overall statistics or just a lack of recent games that have it, it probably up for debate.