I always found it interesting that Star Wars is set in the distant past instead of the (more obvious) future.
Has George Lucas ever commented why he chose to do that?
The goal was to evoke fantasy, not science fiction. Placing it in the dim and distant past freed Lucas up to create a visual and sound palette that was unique, without incurring the ire of 'hard-science-fiction' fans.
RS: You firmly establish that at the beginning of Star Wars with the words: “A long, long time ago in a galaxy far, far away…”
Lucas: Well, I had a real problem because I was afraid that science-fiction buffs and everybody would say things like, “You know there’s no sound in outer space”. I just wanted to forget science. That would take care of itself. Stanley Kubrick made the ultimate science-fiction movie and it is going to be very hard for somebody to come along and make a better movie, as far as I’m concerned. I didn’t want to make a 2001, I wanted to make a space fantasy that was more in the genre of Edgar Rice Burroughs; that whole other end of space fantasy that was there before science took it over in the Fifties. Once the atomic bomb came, everybody got into monsters and science and what would happen with this and what would happen with that. I think speculative fiction is very valid but they forgot the fairy tales and the dragons and Tolkien and all the real heroes.
It was part of Lucas's decision to portray the series as part of a science-fiction-based myth cycle. Lucas was heavily influenced by the work on Joseph Campbell, who studied the structure of myths. Luke Skywalker's character arc, especially in the first Star Wars film, was modeled on the traditional "hero's journey" that Campbell wrote about in The Hero With a Thousand Faces. Some excerpts from Campbell talking about Star Wars can be seen here.
There are many specific examples of the correspondence between the structure of A New Hope and the monomyth Campbell wrote about. For example, Luke is trained by a skilled mentor (Obi-Wan Kenobi), who guides him in the early stages of his journey through danger. However, the first time Luke receives the "call to adventure," he "refuses the call"; he cannot go to Alderaan, because he is needed on Owen's and Beru's farm. Only after his home is made unsafe does he follow Obi-Wan across the "threshold to adventure," traveling outside the delimitations of his previous world; at the Mos Eisley cantina, he encounters the "threshold guardians," who wish to block his advance. He is too weak to defeat the guardians on his own; only his mentor's supernatural powers can beat them back.
Besides a plot rooted in the hero's journey, George Lucas gave Star Wars some other trappings of a myth. Making the first movie "Episode IV" was part of this (although Fox would not allow him to include the episode number in the first release); it gives the sense that the film is just one part of a long, ongoing story—part of a mythic cycle that stretches off both before and after that particular story.
The opening text, "A long time ago, in a galaxy far, far away....," serves a similar purpose. It places the story back long ago, in the half-remembered time of myths. Traditionally, most myths take place fairly early in the imagined history of the world, when magic was a more important part of existence, wielded by great heroes and villains whose power could never be matched in the later days.
It's a common storytelling device.
In English we use 'Once upon a time...' to set the scene somewhere distant in time.
In traditional Japanese folk stories we use 'Mukashi mukashi aru tokoro ni...' which translates as 'A long long time ago in a certain place....' This sets the story in a distant time and a distant place. This is so often used that these types of stories are called 'mukashibanashi' or 'stories of mukashi.'
Lucas has (perhaps deliberately) borrowed the idea of these phrases and changed it to evoke a place far away in distance and time — allowing him to create whatever he wants where noone can challenge his history.
Star Wars is "Mythical" style story telling, sword and sorcery fantasy that just so happens to take place in a pan-galactic society where space travel is ubiquitous.
Unlike most science fiction where technological progress and new developments are often the plot; the Star Wars universe is already a professed and ancient society whose expanded universe goes back tens of thousands of years.
To add a sense of cinematic realism, George described and had ordered designed a "Used and Ancient Future" where machines, environments and settings were weathered, utilized and rusty. The villains live in clean, sterile, colorless settings where as the protagonists are ethnic and culturally diverse to enjoy residence and headquarters in a variety of settings.