Star Wars: The Last Jedi - 39 Things We Learned From The Director's Commentary

18. The Flashbacks

Johnson knew early on that he wanted to do a flashback, but was also aware that there are rarely flashbacks in Star Wars, so it was breaking from the norm. The idea was to go back and see Kylo and Mark when they were first training, but then the question became of how much do they put on screen? "Do we show a full training montage? A scene where we show his corruption?"

It was "too much of a tangent" to see all of it, so instead the decision was made to just go back to the moment that matters specifically to Luke's character, which is the moment of his failure, as it's his feeling that it all comes down to this. The Roshomon-style three versions, however, came late in the writing process. The one from Kylo's point-of-view is the truth from Ben Solo's perspective, and it was Luke distorting things the first time around.