It's because of his mastery of sorsue (sorry for my bad spelling.) Well if you want the ACTUAL reason it because in the original 3 films the Green Light Sabre didn't exist until after Obi Wans death.
This rebellious Jedi master had a different bladed saber than his padawan, featuring a green blade as opposed to blue. However, Obi-Wan's hilt was directly designed after that of Qui-Gon's.
This would be the lightsaber that he used to fight Darth Vader on multiple occasions before ultimately dying with it in his hands on the Death Star. The fact that Obi-Wan's kyber crystals were repeatedly blue suggests that his energy matched up with the colour blue.
Luke Skywalker's lightsaber was changed from blue to green when they were filming because it was difficult to see. This became apparent during act one of Return of the Jedi when the battle on Jabba the Hutt's sail barge was shot during the day. The blue lightsaber blended in with the clear blue sky.
Built to replace his father's lightsaber, Luke Skywalker's own lightsaber contained a kyber crystal and emitted a green plasma blade. Its hilt was similar to that of Obi-Wan Kenobi's third lightsaber, as Skywalker modeled his weapon on his late Jedi Master's lightsaber, but simplified some of the design elements.
Anakin Skywalker carried this blue-bladed Jedi weapon throughout the Clone Wars. After dueling his former apprentice on Mustafar, Obi-Wan Kenobi took Anakin's lightsaber and kept it on Tatooine for nearly two decades.
Leia Organa's lightsaber was a blue-bladed lightsaber powered by a kyber crystal in the core of the hilt. It was silver and copper in color and featured mother-of-pearl inlays. Organa's weapon was considered a work of art, having an elegant symmetry with silver and gold hues that evoke to her upbringing on Alderaan.
The rarest lightsaber color is black and is known as a Darksaber. Tarre Vizsla created it – the first Mandalorian ever inducted into the Jedi order. For years it was kept safe at the Jedi Temple.
After defeating her grandfather, Emperor Palpatine, and ending the Sith threat (at least for now), Rey journeys to Tatooine. There, outside Luke's childhood home, she buries his and Leia's lightsabers, before igniting her own, crafted from the staff we first saw her wield on Jakku.
Master Yoda has risen above all others to claim the top spot as the most powerful Jedi of all time and the true chosen one according to IGN's audience. He won pretty handily as well, as he was victorious in 89,756 of his 95,243 battles and had a win percentage of 94.2%.
Affiliation. This blue-bladed lightsaber belonged to the Sith Master Darth Sidious after his resurrection on Byss. Darth Sidious using the lightsaber to battle Luke Skywalker. In his lab on the Imperial throne-world, Darth Sidious kept a number of stolen Jedi lightsabers if he needed them.
Originally the weapon had a blue blade, but after his destruction of the Jedi Temple and fall to the dark side in 28 ABY, Ren modified it into one with a red blade.
Rey's lightsaber was a yellow-bladed lightsaber that was constructed by the Jedi Rey Skywalker following the First Order-Resistance War.
Yoda's lightsaber was smaller than a standard Jedi lightsaber, befitting his shorter stature. But its green plasma blade was every bit as deadly in combat.
Green lightsabers have been wielded by a number of famous Jedi including Master Yoda, Qui-Gon Jinn, and Luke Skywalker. In Return of the Jedi, Luke's lightsaber color switched from blue to green, possibly representing his development as a skilled Jedi warrior.
Without a doubt, the most notable Jedi with a purple saber is Mace Windu, portrayed in the prequel trilogy by Samuel L. Jackson. Audiences first got a glimpse of the unusual blade in Episode II: Attack of the Clones, making Windu the coolest Jedi in one cinematic moment.
A crystal embodies the Jedi as the heart of the lightsaber, and pure kyber has no color until it's bonded to the Force user. This process likely resulted in Rey Skywalker's lightsaber being yellow.
Rey reforged her lightsaber during the First Order-Resistance War, so when she did it, maybe she picked a yellow lightsaber because she wants to carry on the legacy of the Jedi Sentinels from ages past? After all, the Jedi Sentinels were chosen from the best of the best of Jedi Knights.
Leia's lightsaber is also passed on to Rey, who uses it to help defeat Palpatine, given Leia another role in the defeat of the Emperor and salvation of the galaxy, before it too is buried on Tatooine.
Arguably, some of the rarest types of Jedi were the so-called Iron Knights. These are members of the Shards species - crystals with a brain of their own. They were Force-sensitive and all trained under the tutelage of Jedi Master Aqinos. The Shards had droid bodies, similar to that of General Grievous.
While there are many one-of-a-kind lightsabers in the Star Wars universe, there is only one with the history and legend of the Darksaber, which was created by the first Mandalorian Jedi named Tarre Vizsla.
After the duel, in which Padmé was victorious, the weapon was wielded by Skywalker for the rest of the Clone Wars. Padmé used this lightsaber for many battles to come.
After losing his father's lightsaber on Cloud City, Luke Skywalker constructed a replacement with a green plasma blade, its hilt similar to that of Obi-Wan Kenobi's weapon.
The lightsaber's amethyst color was chosen by the actor portraying Windu, Samuel L. Jackson, who wanted a purple lightsaber because he was "the second baddest Jedi in the universe" and so he could find himself among the many Jedi featured in the Battle of Geonosis scene.