Marketing, merchandising and "universe" details.
Star Wars became part of pop culture by saturating public awareness with marketing messages and merchandising. As everyone knows, no movie had ever been so heavily merchandised prior to Star Wars. Kids played with "action figures", traded cards with still from the movies, built models of X-wings and droids, and read behind-the-scenes books about the special effects. (I did all of those when I was 9!)
Plus the movie included tips of so many icebergs. Who are the Sand People? What goes on in Jawa culture? Where do all these droids come from? What are the different purposes of the different droids? How cool would it be to drive a land speeder? Can everyone in the Mos Eisley Cantina communicate with each other? (And all those are just on Tatooine!) So the entire movie stimulated the imaginations of kids watching it.
If there was marketing for a dull movie, it probably wouldn't have gotten as popular. If there was no marketing, even though the movie was great, it might have become a cult hit but not gone beyond that. It was a combination of the two ... along with the teaser that this was episode 4 of 9!