It’s pretty sad. I’m pretty sure it’s because of the crazy high skill-cieling. New players come in, get their cheeks pounded, then leave because getting good at the game is a lot of work. The number of skilled players grow and the number of lower-skilled players shrinks until the game is all but dead.
I still play Half-Life’s multiplayer occasionally. Quake 1 was always my favorite, but now the closest games we have that aren’t dead are Quake Live and TF2 :/