The original Star Wars was huge without extreme marketing. The original trailer was kind of blah, with ominous music (not John Williams’ later upbeat fanfare). There wasn’t a big buildup in advertising to the film’s release (although they did manage to get an impressive cover blurb in Time magazine – “The Year’s Best Film”. And the Boston Globe ran a bottom-of-the=page blurb on the front page on the day of its release). They didn’t even have all the usual toys ready months in advance (toy manufacturers were caught off-guard by the film’s immense popularity, and it was a couple of months before any sort of Star Wars toys showed up in stories). About the only thing they had out in advance were the novelization by “George Lucas” (actually Alan Dean Foster, as everyone quickly surmised) and the first issue of the Marvel comic book adaptation (which had Darth Vader’s mask and helmet as green with red eyes – https://comicbookrealm.com/series/2036/27448/marvel-comics-star-wars-vol-1-issue-1 )
Yet the premiere was packed, and tickets virtually impossible to get for the first show. And it was released May 25, 1977 – a Wednesday, not even a Friday. I went to an afternoon showing the next day and sat through it twice in an almost empty theater, but that changed rapidly. Star Wars continued to play, without any interruption, at the Charles Street Cinema in Boston for over a year.
So the first film succeeded pretty much on its own merits. Lucas was known for his somewhat obscure science fiction film THX-1138 and a little more for American Graffiti (although I think people remembered the film, not Lucas’ name). The big summer movies were supposed to be the Bond film The Spy Who Loved Me and Disney’s The Rescuers and the WWII film A Bridge Too Far and Smokey and the Bandit. The other fantasy film (which was kinda big in London, where Star Wars opened late in the summer) was the penultimate Harryhausen film Sinbad and the Eye of the Tiger.. All of these films had been advertising heavily in advance of opening. Star Wars blew them all away. I know a guy who saw it 39 times that summer. I think I only saw it seven times, myself.
You can say that advertising was largely responsible for the later films, but i don’t think it’s entirely true – a lot of it was the memory of how good the first film had been. When they re-released Star Wars the next year (despite its still play in some places from the initial release), a lot of the interest was because they were showing the advance trailer for The Empire Strikes Back. I suspect more people went to see the film just to see the trailer than to re-watch the original.
As I’ve written before, I think that Star Wars’ popularity derives a lot from its dive-right-in no-holds-barred embracing of science fiction tropes. Although there had been a lot of science fiction movies and TV series (most notably Star Trek and 2001), moviemakers were wary of them and seemed to think that science fiction concepts were too “far out” and not likely to be embraced by general audiences. Although there was clear interest in a Star Trek movie, the concept had languished for years. Harlan Ellison even wrote an article predicting that such a movie would never be made. Without Star Wars, that might have been true – the closest to a Star Trek revival they had was a Saturday morning animated show and the start of development of a new TV series. When Star wars was a huge hit, the Star Trek movie got greenlit in a hurry, and they cannibalized the incipient TV show for parts. Star Wars showed that there was definitely a popular market for this kind of thing (rather than cheapo flicks like Moon Zero Two or Damnation Alley, or “prestige” stodgy films like Marooned.)
Star Wars gave us dogfight battles in space, the multi-aliened Spaceport Bar, planet-destroying weapons, and Evil Galactic Empires. This was old stuff to fans of science fiction literature, most of it dating back to the 1920s and 1930s work of Edmond Hamilton and E.E. Smith and Leigh Brackett (who Lucas got to write the first drafts of Empire Strikes Back), and kept up ever since. To most of the movie audience, though, all of this was fresh and brand new stuff, since they movies never showed it before.