In an interview with Yahoo, VFX supervisor Roger Guyett said that it was never part of the plan to have Force ghost Ben Solo appear alongside Luke and Leia. The VFX team never even created a Force ghost version of the former Kylo Ren.

Guyett also went on to say that an on-screen cameo by Anakin Skywalker’s Force ghost was never in the cards too. That said, we do get to hear Anakin’s voice (Hayden Christensen) along with the voices of the other fallen Jedi during the final battle between Rey and Palpatine, when Rey was on the brink of imminent defeat. “I thought that was a really fabulous moment,” said Guyett. And if you’re wondering how Force ghost Leia was created, Guyett adds that it’s the same as all the other Leia scenes in the film: footage from Star Wars: The Force Awakens.  Star Wars: The Rise of Skywalker is directed by JJ Abrams and is currently playing in Malaysian cinemas. 

Star Wars  The Rise of Skywalker   Why Only Two Force Ghosts Appeared - 25