Left 4 Dead 2: A Late April Fool's Joke?

[Bacn]

They COULD be, but they could ALSO be done through a $50 sequel. As a company, Valve saw a way to make a pretty penny off of a hit online survival horror game, and is rolling with it. Now, what Valve SHOULD do is learn to understand the meaning of episodic gaming, but that's another rant for another time.

But why would they decide to do this with Left 4 Dead and not Team Fortress 2? After all, they've added quite a bit to Team Fortress 2. New Maps, new game mods, unlockable weapons. yet all of a sudden they decided that Left 4 Dead should have a sequel? Why do this with one franchise and not the other?