Because he had no interest in making more films. He'd fulfilled his promise to make the first three episodes and gave up on making episodes VII, VII, and IX when he made Return of the Jedi.
Hamil has told the story multiple times that Lucas asked hiim if he'd like to play Luke again, as an older Luke handing the torch to a new generation. Hamil asked when that would be and said Lucas said sometime about 2011. Lucas had plans to make more than just the original trilogy.