Games are about entertaining, Playing a co-op game with a few good friends can make a huge difference in how much fun is derived from it.
They obviously knew this, it was clearly apparent in alot of reviews and all the feedback for the first game, everyone loved the co-op. With that knowledge they still decided not to include it in 2? In fact they tried to hide that decision if i remember correctly, as if they knew it would piss alot of people off.
Any game maker that would knowingly take away that much more "entertainment value" out of their product isnt getting a penny from me.