There's a lot to it. There are a lot of elements of sci-fi, westerns, etc. George Lucas studied a lot of Joseph Campbell (i.e. great mythology historian) and used common mythological archetypes for his characters to help make the story resonate.
On top of that, it was way ahead of it's time in terms of special effects and the scope of storytelling. You hooked an entire generation of folks, pretty much everyone in Gen X, to the story in some way. Maybe it was the movies, action figures, cartoons, books, etc. I don't remember growing up playing much of anything other than "Star Wars" from the ages of 4-10.
As that generation got a little bit older, they dove into video games and novels that expanded the universe significantly.
So, from my perspective, as someone that's been immersed into the Star Wars universe outside of the movies, it's more than just movies. There's just so much to explore that you always find something new (i.e. Darth Jar Jar).
