In Star Wars: The Empire Strikes Back, the climactic battle between father and son resulted in Luke losing his iconic blue lightsaber to the depths of the planet Bespin. After its mysterious return years later, Luke refused to keep the lightsaber while presumably still having his green one. However, this has fans questioning why Luke returned to his blue lightsaber for his confrontation against Kylo Ren in The Last Jedi.
After losing his hand, Luke's lightsaber was replaced by his green one in Star Wars: Return of the Jedi. Chronologically, Luke appears next in The Mandalorian with his green lightsaber, and then later confronting a young Ben Solo with it during a flashback during Star Wars: The Last Jedi. From then on, the green lightsaber hasn't been seen, though nothing suggests Luke no longer has it during the sequel trilogy.
However, Rey eventually gets her hands on the original blue lightsaber from Maz Kanata in Star Wars: The Force Awakens, although exactly how she found it is still unexplained. Rey then went on to give the lightsaber to Luke, who promptly threw it away and refused to take it back. Keeping it for herself, Rey had the lightsaber destroyed after confronting Kylo Ren, only to have it repaired in Star Wars: The Rise of Skywalker, using it to destroy Palpatine once and for all.
Despite Luke's resentment toward the blue lightsaber, it's the one he chooses for his final fight against Kylo Ren as he Force-projects himself on the planet Crait. While a specific reason isn't given, fans have some convincing theories as to why he chose this lightsaber over his green. Looking at it from a filmmaking point of view, some believe it's meant as a hint to Luke's Force projection. Subtle clues are given to show that Luke isn't really there, such as the ground salt not moving as he walks. Therefore, seeing the blue lightsaber that was just destroyed gives first-time watchers another detail to look out for.
Another popular theory is that the return to the blue lightsaber symbolizes Luke's growth. Rey offers the lightsaber to him twice before leaving, but each time Luke declines it due to the memory of his failure. Yet, after Yoda tells him to grow from his failures, Luke realizes his mistakes and returns to his blue. This is further backed up by his actions in The Rise of Skywalker, as Luke stops Rey from throwing away the lightsaber and admits he was wrong.
It could also be that Luke is done using his green lightsaber altogether, as it was the one he used when he thought about striking young Ben Solo, so he could see it as a relic of his lowest point. But either way, it's satisfying to see him wielding the legacy-saber one more time, especially since it's unknown if Luke will be seen in live-action ever again.