It appears that Hamill just doesn’t believe that the Luke Skywalker that he knew from the original trilogy would have reacted to the situation that way.
Mark Hamill has made no secret of the fact that he wasn’t entirely sold on the version of Luke Skywalker that Rian Johnson created for Star Wars: The Last Jedi.
Now that the movie is out, Hamill is speaking in more detail about exactly what the issue was.
He feels that Luke would have done more to try and fix his mistake, rather than hiding out on a distant planet.