Bad Jim said:
I'd say what it really comes down to is network effects. They are popular because they are popular. A bigger playerbase means it's easier to find games online, there are more guides to help you play better, there are more forum discussions about it so you're more likely to hear about it and try it. If it's still selling many copies it will probably be supported by official patches too. And of course, you're more likely to have a friend or two that play it.
Games achieve that status in one of three ways:
1) They are early examples of their game type, ideally the first. Street Fighter 2 was the first brawler where you had a big selection of characters. Counterstrike was the first of its type. Dota was the second MOBA. Being free helps, but only if the playfield is empty.
2) Marketing. As a general rule, if you hype the hell out of a game you'll probably get a large number of players quickly as long as it's not terrible. That's why EA has a huge marketing department. It is impossible to be unaware of when the next COD or Battlefield is being released, there are ads on the sides of buses and everything.
3) Luck. Starcraft was one of many RTS games of its' time, but some TV company in South Korea decided to televise it and it became huge there. They could have televised Age of Empires or Total Annihilation and made those huge but they picked Starcraft.
Of course, being good games also helps, but I don't think it's as important as it should be. COD always seems to be popular despite hoorible mechanics like killstreaks.
Good post! You're absolutely right about the popularity snowball effect. The RTS industry goes a certain way, produces Starcraft, continues to evolve while Starcraft becomes a phenomenon, then a large portion of RTS's turn around and start to mimic Starcraft.
The only one point I'd disagree on is marketing. I think there's an inflationary effect to a highly marketed game. It draws in a ton of players very quickly, but just as quickly loses those players to the next thing. Games like Starcraft, DOTA, Counterstrike, all developed their following before marketing became a large factor. The next big EA or Infinity Ward or whatever multiplayer game will be something on everyones radar and it will draw in a huge player base. But I don't see people enthusiastically playing those games for years. Instead, people will hopscotch from one game to the next.
The type of game I'm talking about is the multiplayer game that catches on like a sport. Basketball and football wouldn't work as well if the rules were dramatically changed every two years. COD players seem all too eager to jump from one franchise entry to the next, while you still have people playing CS 1.6 and Brood War. It's also the reason Starcraft 2 is such a conservative update to the franchise, and why LoL is still about minions, lanes, and bases.
I suppose the right place and the right time (and the right price and the right game engine

) are probably the biggest factors in popularity.
What do you guys think of esports venues like Twitch.tv? I'm loving the hell out of their NS2 and Starcraft offerings (getting to hear TotalBiscuit host entires tournies is a beautiful thing), and I always thought if Tribes: Ascend had a bigger presence on it it would have gone a lot further.