Here's a 1977 Rolling Stone interview with Lucas.

I think it's incredibly important to look at how much Star Wars draws upon older pulp sci-fi, westerns, swashbuckling adventures, and war movies, which Lucas openly admits to in that interview. Lots of Star Wars isn't new, it just did old stuff in a very new way. Kids who grew up being forced to watch boring Westerns by their dads suddenly got an exciting one with bad guys in black helmets instead of hats. Kids who loved Errol Flynn adventure movies but found them old-fashioned got laser sword fights. I think its audience in the late 70s was craving all the good parts of many genres that had fallen out of popularity (I don't have a source but I'm sure you can find charts of box office numbers in 1975 and 1976 and show that these sorts of movies were out fashion).