Sometimes they run out of time or there simply isn't a reason to include a gun. Just look at Black Ops and their plethora of useless clones. There's no point if the guns play the same. You want diversity or people get bored of playing. That's why some guns (like the Dragunov) don't make it into the game.
Where will be even more guns in the final game then we saw on CoD XP. For example the M240. At the moment we can`t tell if guns like the Ak-74u, Vector, MP5K, M21ebr, RPD are just in the SP or maybe even survivalmod only. I think that some of them will be included later in the game. Or maybe all that we saw at CoD XP was just a kind of a BETA version of MW3.