I think it's down to several reasons.
Back in the day, online play was restricted to PCs only and was accessible by much lower number of people. Consequently, when you played Counter-Strike, you'd join a server and you'd tend to see the same faces every time you played and you'd begin to strike up relationships. Once you made that connection, you might progress to having an IRC chat room for your group of friends and you'd all connect to it and spend time in there.
I think this method of play with the smaller number of players and the centralised server-based community is the reason why so many communities started up back then. It doesn't happen with Xbox Live because the games are all individually hosted so you never get to meet the same people twice and I would argue that most people wouldn't hit it off based on a single play session with a bunch of strangers.
Secondly, I'd argue the following:
* The PC was the only online gaming platform back in 1999 when CS came out
* Most PC gamers would have bought Half-Life since it was such an amazing game - therefore, there was a huge user-base already there for Counter-Strike when it was released.
* Counter-Strike was FREE - people had nothing to lose if they downloaded it because it wouldn't cost them anything.
* It was one of the first titles to offer something more than the traditional deathmatch game model. Therefore, you'd most likely get everyone downloading it and playing to see what it was like.
I think that CS was a landmark release since it had the right amount of time and under the right conditions for communities to develop and establish themselves and there was nothing to seriously rival it for a good few years. It was also kept alive with frequent beta updates that kept the community wanting more and to continually evolve the gameplay with new features without spoiling the core values.......sounds like World of Warcraft perhaps?
Until there is a definitive landmark title with mass appeal, people will keep moving on.
