ovoon: Well if it did it wouldn't make much sense. It's based off of a multiplayer only mod from a multiplayer only game. Killing Floor is quite the small game. It's meant purely to be a simple co-op shooter in the style of Left 4 Dead. I don't think a campaign really fits this type of game. Single player doesn't belong in everything, just as multiplayer doesn't either. CSS and Killing Floor should stay multiplayer only IMO.
GameRager: When you make a multiplayer only mod then fine it should stay MP only but when you charge full price for it and call it a game IMO it should have some sort of SP aspect....heck even L4D has a better "sp" mode, complete with progressive map set and some storyline.
bansama: Killing Floor has a solo mode. In fact the only class not really supported by singleplayer is the medic as that requires you heal team mates to rank up. But asides from that, the game can be played solo with out any real problems.
GameRager: As I told ovoon it's not true SP mode as you can pick random maps and classes and level up I think but that's it. L4D did it better imo, with various semi-linear level sets you could plow through which also had some bits of story in them as well in the character banter and such.
But it never was full price. It came out as a $20 game. Like an indie game. A game with only one mode, which is very well done. No campaign needed. And some tacked on SP to killing floor wouldn't have been a good idea.