I think the first factor is how well known Star Wars is. It's universal. It's part of our very culture at this point. Because it is so well known it makes parodies of it accessible to a large group of people. Sure, I could parody "Dan in Real Life" but who would care? A much smaller segment of the population than would care about a Star Wars parody, for sure.
Another factor is passionate fans. I mean, PASSIONATE. On "How I Met Your Mother" Ted's friend Marshall tells him to break up with a girl because she doesn't like Star Wars. If you run into somebody that hasn't seen Star Wars you say, "WHAT?!?! How can that be?" Fans discuss every aspect of every character and every bit of trivia and minutia is analyzed to the tenth degree... which makes a parody so interesting for fans. Sometimes the source of the humor is something you never considered before, such as, "What would happen if a Jedi dropped a lightsaber on a spaceship?" (Robot Chicken provides the answer.)
Finally, because of the fantasy / sci-fi aspects of Star Wars it allows great room for jokes. Because we're dealing with alien races, spaceships, and something called "The Force" it's a little absurd to begin with. So, naturally, this can be exploited for comedic effect in a parody. You don't ever seen a parody of Schindler's List, do you now?
Parodies of Star Wars are done with a fun spirit by people who love Star Wars and it is clear by their reception that fans receive them in this spirit. It's part of the fun of being a Star Wars fan.