https://screenrant.com/star-wars-luke-skywalker-black-costume-meaning-reasons/

Luke Skywalker appears on The Book of Boba Fett still wearing black even though it's been five years since the events of Return of the Jedi — why?

really hard to find a decent dry cleaner on Tatooine :(

I love all 3 movies :)

When I saw in the preview for the Abrams Add-Ons that Luke Skywalker was not only in black but had made ZERO efforts to repair his prosthetic right hand that had gotten damaged on Jabba's Barge in ROTJ, but had let it devolve into a rusty metal CLAW, that's when I decided to PASS on the whole deal and NEVER watch that depressing set of movies. Sorry but the Original Trilogy Luke would have been FAR more positive and resiliant instead of being so totally sulky and self-pitying!

But then again, ever since 'Lost', I've NEVER trusted Abrams to NOT to turn any kind of plot into a dreary, depressing and sadistic mess.