To be honest, I don't see how anyone thought this game would do 'fantastic'. It's doing well, to be sure, but the first game got 'somewhat-above-average' reviews, and likely only sold as much as it did (still not a huge amount) because it was the stand-out title of the PS3's launch.
Call of Duty was always going to do well--it's friekin' Call of Duty! XD