Because Star Wars will still be Star Wars no matter what they do with it. Having more than one movie a year would drain all enthusiasm from the product rather quickly. Marvel on the other hand is so varying in content and characters and situations that it can get away with more than one movie a year. Thor is different enough from Captain America who is different enough from Iron Man who is different enough from Ant-Man who is... you get the idea. Having more than one Star Wars movie a year would be like having an Iron-Man movie every single year. It would diminish the product.