The star wars theme was written purposely on the same key as the 20th Century Fox fanfare (that used to appear before the movie started) so that there will be a continuous feeling and a smooth entry to the film for the viewers.

Why Disney Should Add Fox’s Fanfare Back to ‘Star Wars’

Here’s a magic trick. I can make you hear something just by reading four words: 20th Century Fox fanfare.

If you’ve seen a movie in your lifetime, chances are your head is now filled with snare drums, timpani and orchestral horns. There’s also a fair likelihood that after that 20-or-so-second phrase drifts away into the foggy ether of your mind that another tune might creep in on its heels: the Star Wars theme.

Perhaps no film score in history is as iconic, and certainly no studio’s opening credit tag evokes such visceral reaction as that of Fox’s short march. The two themes are intertwined, stapled together by decades of Pavlovian reinforcement as deep-rooted as the soft jingling of sleigh bells kindles the image of jolly old Saint…
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Source: https://www.hollywoodreporter.com/heat-vision/why-disney-should-add-foxs-fanfare-back-star-wars-1067900