Obi-Wan Kenobi knew that Anakin Skywalker became Darth Vader, so why didn't he try saving him from the Dark side in Star Wars? As master and Padawan, the pair went through so much together, and while the prequel trilogy didn't dive as deep into their brotherly bond, Star Wars: The Clone Wars animated series made this clear. Considering all the time they spent with each other, it's curious Obi-Wan didn't try to convince his former apprentice to return to the Jedi path.
Originally poised to be the Chosen One to bring balance in the galaxy, Anakin's life was defined by tragedy in Star Wars that by the time he secretly married Padmé Amidala, he was so crippled by the fear of losing her, exacerbated by his nightmares and visions of her death. Emperor Palpatine took advantage of his desperation for control, and slowly manipulated him with the intention of making him his apprentice. This all happened in front of the Jedi Council, who were blind to the villain's ministrations which made everything worse. By the time they figured out Darth Sidious' plans, Anakin had already pledged his allegiance to the Dark Lord of the Sith.
Still, it's clear Obi-Wan cared greatly for his former Padawan, even in his last moments. During their confrontation on Mustafar, he was heartbroken seeing what happened to Anakin. Even when he had the chance to end him then and there, the Jedi Master couldn't bring himself to kill him. So instead, Obi-Wan left him limbless, not expecting Palpatine would come to rescue his fallen Padawan. It's not yet clear when in canon Obi-Wan learned Anakin had become Darth Vader, but he did before the events of A New Hope. Given this, it's curious why he never tried to appeal to Anakin's humanity again years later after their falling out. Looking at things from Obi-Wan's perspective, however, there a couple of reasons why he didn't go this route. For starters, this would've meant showing himself to Darth Vader and essentially the Empire. There's a reason why he and Yoda made sure to separate and hide in the most unlikely planets — they didn't want to be found. It was necessary for their survival and knowing full well that they're outpowered by Palpatine, doing this would've posed such a big risk on both their safety.
Secondly, based on their last encounter on Mustafar, Anakin was obviously too far gone to the Dark side as he had just gone on a killing rampage in the Jedi temple. Obi-Wan thought that he's far beyond redemption; in hindsight, not really understanding what drove Anakin to the Sith didn't help the situation. At the very least, Obi-Wan knew he wouldn't be the one to convince him to come back to the Light side since Anakin thought that it was his Jedi master who turned Padmé against him. So instead of trying to convert Anakin, the Jedi Master focused his time and effort on making sure that Luke Skywalker was safe in Tatooine, hoping that one day, he would be the one to defeat the Empire and his father.
Lucasfilm is about to reveal further information about Darth Vader and Obi-Wan's relationship after the Star Wars prequel trilogy in the upcoming Obi-Wan Kenobi series on Disney+. Hayden Christensen is confirmed to reprise the Dark Lord and the show will see him battle it out again with his ex-Jedi master. While Obi-Wan decision to not convince Anakin to renounce the Dark side after he became Darth Vader is justified given what's known thus far, perhaps the show can give a definitive answer to this query.
