The Star Wars main theme actually was developed as Luke’s theme and is used as such in the original trilogy.

Yeah I really wish the ST had a more consistent musical style. For example, I really liked the overall feel of the soundtrack in The Last Jedi. Despite overusing some old themes, the new compositions had a really interesting style that had a mixture of elements thrown in from the originals and the prequels. Yet no theme that debuted in TLJ was kept for the last movie. For the sake of comparison, both Empire and Attack of the Clones introduced at least one key musical theme (Imperial March, Yoda’s theme, Across the Stars).