How can the title of the story "The Canterville Ghost" be justified?

The title of this story is justified because it really is a story about Sir Simon, the Canterville ghost. When the story begins, the reader thinks that this is a story about the Otis family and their move from America to an aristocratic mansion in England. As the story progresses, however, it becomes clear that this is, in fact, focused on the Canterville ghost himself. The narrator frequently mentions his haunting career, for example, and his numerous tactics to frighten the very skeptical Otis family. Much attention is also paid to the emotional state of the ghost, as his haunting ability is mocked by the Otis family.
This idea is further reinforced in Chapter Five when Virginia Otis and the ghost share an intimate conversation. The ghost reveals the truth about his marriage and confesses to Virginia that he is desperate to go to the Garden of Death where he can sleep in peace forever. The story is, therefore, transformed into a tale of the ghost's redemption, as well as the very unlikely friendship which he develops with Virginia.

I believe that the question is asking why this story is called "The Canterville Ghost" instead of having some other title. I understand the question, and it is a good question because the title of the story could absolutely be focused on the Otis family. The title could have been something like "The Otis Haunting."
"The Canterville Ghost" as a title makes the most sense to me because Sir Simon's ghost is the most interesting character in the story. The Otis family is hilarious, but ultimately the Otises are flat characters. They are the same throughout the story, and that is also true of Virginia Otis. Having the title focus on the Canterville ghost helps focus the reader's attention on that character. Sir Simon is by far the most interesting and dynamic character in the story because readers get to see a wide range of his emotions. We see him exuberant and overconfident in his abilities to scare off new homeowners. We see him angry at the Otis family and even scared of the Otis family. By the end of the story, most readers will even sympathize with Sir Simon's situation. The title, "The Canterville Ghost," is an effective title because it is a short, to-the-point title that correctly focuses reader attention on the most important character.