Warning: SPOILERS ahead for Star Wars: The Rise of Skywalker.
Star Wars: The Rise of Skywalker? concludes with the Force ghosts of Luke and Leia Skywalker watching over Rey - but one Force ghost we don't get to see is that of Ben Solo a.k.a. Kylo Ren. In the final chapter of the Skywalker Saga, Kylo Ren completes his redemption arc and reconnects with his identity as the son of Leia and Han Solo. He sacrifices his own life force in order to bring Rey back from the dead, shortly after she saved the galaxy from the destructive powers of a revitalized Emperor Palpatine. Audiences witnessed a lifeless Ben become one with the Force, with his physical form dissipating into nothing as he allowed the light to set him free from the darkness.
In the closing scene of the film, both Luke Skywalker and Princess Leia Organa-Solo appear to Rey as Force ghosts at the Lars homestead, but Leia's son - the last of the Skywalker bloodline - doesn't join them. One would think he would appear to Rey as well, given that his self sacrifice was not too dissimilar from that of his grandfather, Darth Vader. After all, at the conclusion of Return of the Jedi, Vader did appear as a Force ghost in his Anakin form to Luke during the Ewok celebration. But despite all of the Force abilities on display throughout the film, the ability to return as a Force ghost is one that Ben either cannot or does not exercise.
In Star Wars canon, the ability to become a Force ghost does not come naturally. It is not something that one is born with or can do as they see fit. It is an ability only the most devoted and in-tune with the Force could achieve, and takes training and careful study in order to achieve. After all it is, in many ways, another form of life. In order to interact with the living in any way beyond death is not a feat most Jedi could ever hope to achieve. It took even the most powerful of Jedi such as Yoda and Luke Skywalker a great deal of time to learn the techniques needed to pull of this ability.
Therefore, it would make sense that Ben did not appear as a Force ghost in Rise of Skywalker because he simply never learned how. The majority of his adult life he spent terrorizing the galaxy as Kylo Ren, seeking out Luke Skywalker, and leading the First Order on their ruthless crusade across the galaxy. He had no time or relative need to learn such an ability prior to his death. Not to mention, it is generally understood that dark side Force users and Sith were incapable of learning such a technique. Kylo's suppression of the light for so many years only made it more improbable that he was even remotely able to learn the ability in the first place.
To sum it all up, while Ben did finally redeem himself and return to the light in the end, his lack of knowledge of how to become a Force spirit is probably what kept him from appearing as such in the final scene of Rise of Skywalker. Though there is still some debate as to how Anakin was able to do so in Return of the Jedi, but the concept of Force ghosts is a rather hazy one at best.
Purely in terms of storytelling, director J.J. Abrams may have chosen not to show Ben Solo alongside his mother and uncle because that moment is about Rey deciding to take up the Skywalker name - and neither of Kylo/Ben's names end in "Skywalker." Leia and Luke were both also mentors to Rey at one time in her life, whereas Kylo Ren was not. Still, even if she couldn't see him, Rey could take solace in the fact that Ben Solo was at peace and was at last one with the Force.