After storming away from an uninvited guest, most of us probably wouldn’t first think that what we wanted to do next was change our clothes.
However, Luke had been standing up there, thinking about what it was he wanted to do, likely trying to build up the courage to take the step he felt was necessary.
Once he realized it wasn’t going to happen, he changed out of his formal clothes, the same we’d change out of a nice suit or dress after discovering the formal event had been canceled.
Why Luke Skywalker changed his clothes might not have been at the top of anybody’s list of questions after seeing Star Wars: The Last Jedi, but it was still an interesting thing to add to the story that might have made some curious.