Tribes is a very easy "drop in, drop out" game. Games don't last long, they are fairly fast paced, and it doesn't take long to load. It's also always populated, modern (though doesn't need *too* great a PC), and FREE. It makes it the perfect game for community events, as it's available for us all, and for people who get bored of that type of game quickly can still have fun in one or two maps before quitting. There's also the fact that a game like this is a great game to bring non-BI friends to, and meet new people, as shown by the last week.
Also, starting to run events for games such as this is by far the most fluid and organic way of turning a "game we like to play" into a "Supported Game".
I know for a *fact* that if I were to organise an FS event for one month from now, giving plenty notice and making sure everyone knows, we'd see less turnout for that then we will for an playdate such as this.