I think people are underestimating the guilt Luke must’ve felt when Kylo turned. Ben wasn’t just any student, Ben was his nephew, the son of his best friend and sister; and Luke failed him. Luke failed Ben in the worst way a Jedi master can fail. He also failed all of his other students, and they died because of his failure.

EDIT: On top of that, he probably figured that Leia and Han never wanted to see him again. Or, at least, he couldn’t bring himself to face them after what he’d done.