The trailer was rushed and approved by George Lucas put together in three days (!!!) to meet the need to have something to show to movie goers during the 1976 Christmas season.
Hyping the film was more important than being accurate at the time. Heck the special effects weren’t even finished so things such as conveying the proper tone/spirit of the film were not a priority at that moment. More details here on a post on the official Star Wars website; pertinent excerpt here with bold emphasis being mine:
With the production in a mad rush to finish the special effects, Charles Lippincott was busy promoting the movie at conventions and by other means like the novelization and the Marvel comic that was in development. One of the other tools in his shed was the first teaser trailer. Lippincott met with Lucas, Kurtz, assistant optical editor Bruce Green and three ad agency people in November 26, 1976, to talk about the trailer’s storyboard and everything that was needed to make a rough cut. This was finished a mere three days later and talks began about what needed to be changed and what music would be heard in the trailer.
In the next couple of weeks, Bruce Green would travel back and forth between ILM and MFE (Modern Film Effects — the company responsible for the effects of the trailer like the exploding logo) to get the teaser ready. It was fully approved by Lucas in early December and ready for review by Fox. However, tensions were high due to the troublesome production that the movie had up to this point, and the executives were far from pleased with a teaser that featured a couple of unfinished shots and was seen by Lucas and his team as a “spirit of the movie” trailer. Fox even went as far as trying to change the title of the movie, but that never happened because nobody ever gave Lucas acceptable title alternatives. The teaser was finally released in cinemas during the Christmas season, and receptions were a bit of a mixed bag with some people becoming curious for the movie while critics said it would never work.
And there are even more details provided via this post on website Episode Nothing: Star Wars in the 1970s.
So at the end of the day the final product of the trailer was fully approved by George Lucas; warts and all. Why? It was clear more important to get something out there to the public during the Christmas 1976 film season than to worry about things such as the soundtrack being off, special effects being in place or even whether key plot points were made with subtlety or not.
Besides, in the pre-video days of the 1970s, trailers were disposable and not something people cared about others seeing in the future. By the time the film would actually be released, flaws in the trailers wouldn’t never be reviewed by casual fans since the they would not have access to view the trailers over and over again as we do nowadays.
Also, the original Marvel Comics film adaptation—which was created way before the film was finalized—shows utterly no indication that “A long time ago in a galaxy far, far away…” was ever in the script. In fact the opening crawl text used in the comic is 100% different from what appeared on screen which was rewritten with the help of Brian DePalma; unclear what script draft this version of the crawl originated from. See image below:
Also, I dug up this scan of the Star Wars program page—seen below—that was a part of the 1976 San Diego Comic-Con that took place July 21–25, 1976. There was a Star Wars slideshow/presentation that was presented there that was not connected to 20th Century Fox’s promo efforts but rather George Lucas’ own grassroots effort. Read the text and you can find the same somewhat hokey—yet oddly specific in places—tone in overall presentation that Christmas 1976 teaser had; transcribed below.
“I have wrought my simple plan. If I give one hour of joy to the boy who’s half a man. Or the man who’s half a boy.” —Arthur Conan Doyle’s preface to “The Lost World”
STAR WARS, a live-action space adventure-fantasy, involves the search for a kidnapped rebel Princess, and a confrontation with the dark forces of an evil space empire.
Through thousands of light-years come to the unusual exploits of hero Luke Skywalker and his friends, flesh-and-blood space pilots and mechanical robots, as they battle numerous villains and creatures in a massive Galactic Civil War. This story has no relationship to Earth time or space. It takes place in other solar systems in another galaxy and could be in the future, the past or the present.
Young Luke Skywalker is accompanied by his robot companions R2-D2 and C-3PO; the tough starpilot Han Solo; the seven-foot, fur-covered Wookie, Chewbacca; and the venerable old warrior, Ben Kenobi.
Three different worlds become settings of the series of colorful adventures and thrills. They travel from the large arid planet of Tatooine to the huge man-made planet destroyer, Death Star, and finally arrive on the dense jungle-covered fourth moon of Yavin.
Director-writer George Lucas has created a majestic visual experience of extraordinary worlds. The Panavision Technicolor motion picture is produced by Gary Kurtz for Twentieth Century-Fox release and was made on locations in Tunisia and at EMI Elstree and Shepperton Studios, London over a 17-week schedule.
Lucas and Kurtz, the successful duo of AMERICAN GRAFFITI, have acquired an outstanding production team, including production designer John Barry of A CLOCKWORK ORANGE fame and director of photography Gil Taylor of Alfred Hitchcock’s FRENCY and Twentieth Century-Fox’s new hit, THE OMEN.