Someday someone will hopefully sit down and explain to me how one of the most beloved franchises in gaming history petered out like a stale fart.
Valve tends to work like an anarchist commune, which means that at a certain point the right people got bored and started working on something else abandoning the project.
They also seem to have lost interest with single player narrative games in general, probably because most people there are game designers and multiplayer games don't have all that pesky story and dialogues that force to consider how game mechanics relate to them.