For the casual audience:
1. Too dark, they don't like games where they can't see anything.
2. The game isn't as pick up and play as other titles.
3. No local multiplayer makes it a harder sell vs games which have this feature.
4. Much of this audience doesn't buy games too soon after each other and it came too soon after Call of Duty 5.
5. Casuals probably didn't rate the graphics as highly nor is it as important to them.
For the hardcore audience:
1. The multiplayer class system / online was a little borked, for example there was a limited number of spawn grenades, turrets etc so if you shared the same class as too many others you couldn't perform the role.
2. The controls were dissatisfying, I heard talk of latency but I dunno about the relationship there.
3. The storyline wasn't particularly brilliant and it had very unlikeable characters, this is generally enough to get a movie rated down for example.
4. Rated highly for graphics, which means gameplay wise its probably mid 80s like RFOM2.
5. The gameplay was a little off and lacked the class of gameplay design that other highly ranked shooters have, it made a lot of little errors which the reviewers wouldn't generally pick up on.
This is my assessment as to why the sales have pretty much died on this game.
