1. Gabe Newell recently discovered he's allergic to large piles of money.
2. Valve's pretty sure Half-Life 2: Episode Two tied up all the loose ends and gave perfect closure to the story.
2.5. Team Fortress 2 hat development is tying up all of Valve's resources for the next decade.
4. They're too busy developing a Half-Life prequel where you wander around the Black Mesa facility, silently running tests.
5. They're pretty sure no one on the internet would want it.
6. Team is still trying to find the perfect voice actor for Gordon Freeman, and Gilbert Gottfried won't return their calls.
7. Gabe Newell's enjoying the "Gabe Newell doesn't believe in the number 3" meme way too much to give in now.
8. The dev team's too busy photoshopping pictures of Bryan Cranston as Gordon Freeman.
9. Valve invented a real Gravity Gun and are having way too much fun playing around with it to sit down and make another game.
10. It's already completed, but they legally can't release it since testers found it to be so good they instantly died from pure joy.