For what it is, it's about as close to perfection as you can get. That is what gives it a longevity that other games have a lot of trouble matching. You see this in all genres, though, for example, I have no doubt that if Starcraft were on here, you would see it showing a similar longevity.
And it's usually #1, when other games aren't being launched...
The two games also combined today for more players than MW3 Multiplayer (likely the long term predominant portion of it), and since I'm assuming Skyrim numbers will eventually die down, it's often still the most played game on Steam.
It has been developed and revamped over time. Counter-Strike today has not much to do with the original HL mod.
The original Counter-Strike is no longer actively developed is it? Yet, surprisingly, it seems to be ahead of Counter-Strike Source, the newest addition.
CS:S has higher requirements, and the gameplay changes pissed off a lot of the hardcore CS crowd. I know people who still kvetch about Valve nerfing deagle-hopping.