Hamill was given the role as Luke simply because he had the "Pop teen hunk" look for the 70's. After Star Wars was released that look was starting to fade and they regretted having him play the role of Luke. If not for Darth Vader, Han Solo (Harrison Ford) and the cutting edge filming, Star Wars would have flopped.
I probably saw Star Wars at least 7 times in the theater, I was a fanatic until after half way through Empire Strikes Back then the entire series went down hill from there. Return of the Jedi was a huge letdown considering how long we used to have to wait between releases. Nearly every scene that is Luke oriented is cringe shit that makes the character of Luke feel like a giant fucking crybaby.