Because shooters have almost no longevity. They're like hollywood action flicks, a new batch is tossed at you each year. You only get a sport when a game doesn't change very much such that competition rules can be understood by competitors and spectators alike) which makes it impossible for shooters to gain and maintain that critical mass fanbase. In order to have a sport you a;sp need a massive and very active playerbase. Most commercial/popular shooters that might make half decent spectator sports are also locked down pretty hard for commercial reasons (eeking you out more $$$ though DLC) - there's no way to mod them, there's no dev community or ability to make competition/sponsored maps. The games that are still moddable are typically too old now (compared to SC2). People either don't play them (they all playing Call of Duty Black Ops 17 or Battlefield 29) or won't simply watch them anymore because they look way too dated.
Also, in order for something to be a sport it must be entertaining to spectators! As most shooters are first person (or near enough) it makes them damn near impossible to observe the entire game. Fans of both teams/players should get a near equal view of the action from a neutral perspective - thus far no shooter game has managed to build in a neutral spectator system that's been worth a damn.
SC2/LOL are much more spectator friendly. The perspective is inherently neutral. It's easier to track the state of the game as it changes, much like other team orientated sports. Also, most of the effects aren't overdone - if you're spectating a player in a shooter and they get shot, the screen is flashing read, getting blured, if an explosion goes off there's dust/debris/smoke flying all over the place, 90% of the time 80% to 100% of the screen will be covered for a few seconds because shooters mostly focus on immersion. If a player in SC2 drops a psi-storm the effect is precise, you can see exactly who/what was hit - it's not a stream of lightning from the sky (which is what most developers would have done because effects like that should be super fun and rewarding to use!) and lots of other tiny details like that that jsut make the game easier to read, track and follow.
Last edited by Drac; 13th August 2012 at 08:07 PM.