Why did Leia disappear after Ben Solo's death? The climactic ending to Star Wars: The Rise of Skywalker featured the deaths of Leia and of her son Kylo Ren, now redeemed as Ben Solo. As Ben Solo's body disappeared, only then did Leia's body also vanish.
Force ghosts have been a core part of Star Wars lore ever since Obi-Wan Kenobi vanished when Darth Vader delivered the killing blow in Star Wars: A New Hope. Other Jedi, like Yoda at the end of Star Wars: Return of the Jedi, became one with the force at the time of their natural death. Luke Skywalker was also shown disappearing at the end of Star Wars: The Last Jedi after using his remaining strength to give the Resistance a chance to survive. The Rise of Skywalker continues this trend with both Leia and Ben Solo's deaths.
Star Wars: The Rise of Skywalker is packed with moments of lore and fan service. Force ghosts were shown once again in the film, with the deaths of many key characters tying in. The death of Leia was an emotional moment for fans and audiences alike, it was also the most expected. With Carrie Fisher's passing, only limited amounts of archive footage from prior films were available to use by J.J. Abrams and Chris Terrio. Fulfilling Leia's arc and providing closure for her character required her to redeem her son, Ben Solo.
As Rey and Kylo Ren battled on the remains of Death Star II, Leia reaches out to her son one last time, giving every ounce of force energy she has left to reach him. Her final word in the film is simply her son's name as she passes away. Kylo Ren immediately drops his lightsaber, snapping out of years of psychological manipulation by Emperor Palpatine. Yet, as Leia dies, her body does not disappear like the other Jedi. It stays until the very final moments of the film, at the point of Ben Solo's death.
Much like how Leia gave up her life to give Kylo Ren the chance to snap back into Ben Solo, he emulated that to give Rey a chance to survive. Using the last of his own force energy to heal Rey of her injuries, he dies in true Skywalker fashion, in sacrifice. The sacrificial death to save another has now become a Skywalker trait, with Anakin Skywalker, Luke, Leia and now Ben all giving up their respective lives to save another.
The question of why Leia's body only disappears after Ben Solo's death has a few potential answers, but it boils down to Leia's own journey as a Jedi. Her character can be considered one of the most important of the sequel trilogy. Her arc has always been inherently tied with that of her son. Leia's arc has also been one of tragedy. She loses Ben Solo to the dark side, then she loses Han Solo when he's killed by their son. Her brother Luke sacrifices his life to save her and the rest of the Resistance. When she in turn offers her own life to save Ben Solo, her body does not leave as her journey is not complete. During her Jedi training with Luke, she experienced a vision of her son's death coming at the end of her own path, and Luke notes that Rey is the one who is destined to complete Leia's journey. That path was always going to lead to the death of Ben Solo. Leia, knowing this, does not completely pass into the netherworld of the Force until she can guide Ben Solo to come with her.
As Ben Solo's Jedi training was always incomplete, and his redemption came so late in his journey, the path to becoming a Force Ghost could have been beyond his knowledge. Maz Kanata's smile as she sees Leia's body pass into the Force only after Ben's death seems to indicate Leia, a true Jedi, knew what her ultimate purpose was in Star Wars: The Rise of Skywalker. It was to save and redeem her son, and in turn, redeem the Skywalker family.