Reason #756,345,233 of why I love Wikipedia. Star Wars Music has its own entry:
The scores utilize an eclectic variety of musical styles, many culled from the Late Romantic idiom of Richard Strauss and his contemporaries that itself was incorporated into the Golden Age Hollywood scores of Erich Korngold and Max Steiner. While several obvious nods to Holst, Walton and Stravinsky exist in the score to Episode IV, Williams relied less and less on classical references in the latter five scores, incorporating more strains of modernist orchestral writing with each progressive score. The reasons for Williams’ tapping of a familiar Romantic idiom are known to involve Lucas’ desire to ground the otherwise strange and fantastic setting in well-known, audience-accessible music. Indeed, much of the trilogy’s success relies not on advanced visual effects, but on the simple, direct emotional appeal of its plot, characters and, importantly, music.