Here's Chris Taylor, author of “How Star Wars Conquered the Universe, in a must-read interview with Joshua Rothman in The New Yorker: 

To my mind, one of the genius things about “Star Wars” is that it was one of the first movies to really say, “This is in no way, shape, or form connected to Earth.” It’s “a long time ago, in a galaxy far, far away.” Even with superheroes, as soon as you set it on Earth, you’ve limited it to one culture or another. But “Star Wars” is irredeemably distant. From that initial moment of genius sprung so much of what we love about “Star Wars.” 

To my mind, too. From my nearly 20-year-old review of ”Star Wars“:

Perhaps the most imaginative thing we see is the first thing we see: The words ”A long time ago, in a galaxy far, far away." This allows George Lucas to come up with anything his imagination desires. He does.

I still remember thinking, in 1977, at the age of 14, how visionary that was. Unlike almost every other attempt at sci-fi at the time, it wasn't the future and it wasn't Earth. It wasn't us. It was somewhere far away and at a time waaaay in the past. That seemed genius to me. Still does. 

Read the whole thing. Most interesting tidbit for me? That in Lucas' Vietnam-era mind, the Empire was the U.S. military, the Emperor was Nixon, and the Rebel force (Luke, Obi-wan, Wedge, etc.) was North Vietnam. Someone alert Rick Perlstein.