At the time of release Counter-Strike didn't have many competitors of similar caliber to compete with. The only comparable widely known products have been Rainbow6 and Rogue Spear. Both of which had more of a single player focus although both quite fun in multiplayer. There is now a huge rush of Counter-Strike inspired clones currently in development by a number of game houses. None of which will probably replace Counter-Strike but have at least hopes of riding the wave.
Team play environment.
People love to feel like they are part of a team and more importantly win with that team. A team play game with a time limit rounds system. Combine that with the way the weapons and players work you are basically forced to stick with your team members for the best chance of success in most cases. Further enforcing the ideology of teamwork and camaraderie among players. The real world.