Developer Support, snowballing Gooseman and Valve Software have put incredible amounts of hours into the development and support of Counter-Strike. In addition to the obligatory bug fixes and game tweaks something else happens. You keep updating your game you keep getting attention. The more attention you get the more people will be made aware of your product. In addition to that of course is the standard snowballing effect.
More people attract even more people.
CliffyB of Epic Games, currently working on Unreal Warfare Counterstrike has a Death Penalty. You die, you're out. It's like when I used to play dodgeball as a kid - knocking someone out of the game was that much more satisfying because I knew they were OUT for the duration of the game. My own status in the game then became that much more of a concern, because if I got hit then I'd have to watch from the sidelines, helplessly. It's also one of the few titles that, when you go online, people consistently play as a team.