This.

If you look at the top 10 grossing movies of all time (heck, you can dive down even deeper into the top 100) you'll see that the list is dominated by big epic adventure and fantasy movies. Star Wars isn't a cold sci-fi movie, it is a fantasy movie ("Long ago..." = "Once upon a time...", lightsabers = swords, The Force = magic) and an epic at that. Star Wars was in a lot of ways the progenitor of that kind of film; if you look at the other top movies from the 1970s you'll see a different sort of film (Jaws, The Exorcist, Grease, The Sting, Animal House, etc.).

Star Wars also came out around the same time as some other fantasy adventure stuff was gaining popularity. Dungeons & Dragons came out in 1974 and 1977-78 was when AD&D split out, and there was I think a Lord of the Rings movie around 1978 too. There was a whole 1970s dark fantasy epic aesthetic thing happening. So it was good timing, like people doing vampire movies when vampires were hot.

You also have to consider that Star Wars was a big deal not just because of the movie but because of the merchandising. A lot of people grew up with the Kenner Star Wars toys, which weren't really like anything they'd had before. The only comparable thing in terms of size, variety, appeal, etc. was the GI Joe toys which came out in the early 80s. Star Wars wasn't just a movie, it was a franchise and a phenomenon.

I think it's also worth considering that people like big feel-good epics with strong heroes during times when things are not so great. The late 70s during the Carter administration in the US were kind of recessiony and gloomy. Compare to The Wizard of Oz which was popular in 1939 (tail end of the Great Depression). People like to feel good when they feel bad. Star Wars was a feel good movie.

John Williams' score also helped. Music is a big part of why the Star Wars movies are so memorable, and Williams had a string of big epic scores around that time (Jaws, Star Wars, Superman, Raiders of the Lost Ark, E.T.) and you can probably immediately call to mind the themes of those movies just from the name.