Luke becomes a hermit, because RJ wanted him to be, not because it flows naturally from previous installments in the saga.
RJ?
You seem to have skipped a movie.
Before TLJ we didn’t know why Luke went to Ach-To looking for the first Jedi Temple. The bleak I came here to die, Jedi must end, being disconnected from the Force, killing my nephew, etc, are all TLJ’s contributions. TFA left it all a mystery. TLJ didn’t really provide any answers for those mysteries, except in the most superficial manner possible. It doesn’t provide much context or motivations for Luke or Kylo. Kylo is just a bad egg, and Luke a far more flawed Jedi and person than we were led to believe, the latter being RJ’s interpretation (or distortion) of the character.