That's because they're working on DOTA & TF2 which are seeing massive updates right now along the lines of new animations, modelling, gameplay balance, writing/voice acting, engine coding, and a whole heap of other things.
DOTA is just about to enter a 2nd beta stage, with two betas ("DOTA 2" and "DOTA 2 Test") simultaneously running at once. I highly doubt that just the DOTA team is working on that.
The TF2 team is working on TF2, I highly doubt they're taking anyone from the L4D team. They aren't inept; if they had to take members away from every other team every time they made a big update, we probably wouldn't have Portal 2 out. And even if the DOTA 2 team had to drag members of the L4D team away, they always leave enough members in a team to still do work on the game. And they've done jack since The Sacrifice.
The L4D team is lazy, even more so than the TF2 team who still manages frequent updates.
Cut the optimism and accept that the L4D team isn't doing anything with the game.