It all boils down to this:
L4D was made by TurtleRock. That’s a group of people who became part of VALVe when they already had L4D almost finished). And they don’t seem to share VALVe’s standard passion for maintaining their games, supporting them with timely fixes and updates. Instead, right after L4D was out, they started working on a sequel. It would seem that they didn’t even consider the option of making it a cheap Expansion pack, perhaps even splitting it into 3 or 4 expansions, each having 2 campaigns and 1 new SI (which would not shatter the community like making a full sequel does).
Plus, they don’t seem to be at all interested in fixing many issues people have with L4D1. Tanks still spawn on fire, Boomer bile still doesn’t stick to some survivors who clearly got a hit…
======
Now, the question to ask yourself is: Do I trust them to support L2D after release? Will they fix issues the game has? Will they remove bugs that are in it? Will they work on online features so that matchmaking is not pain-in-the-ass? Or will they decide to start working on Left3Dead immediately?