I'm not sure I'd call Battlestar Galactica or Stargate big SF sagas on the scale of Star Wars or Star Trek – I don't think they've spawned nearly as much tie-in media. Maybe Doctor Who? But Star Wars is in a league of its own wrt tie-in stuff. It's easier to have good computer games if you have lots of computer games. I guess Star Wars simply hit a cultural zeitgeist and got mega-popular enough to become massively multimedia?