It's a comical story. It probably has some unique Christmas - related situations and characters that make it interesting. Maybe there are some misunderstandings or funny events that occur during the Christmas season in the story.
Some fanfics might focus on the psychological aspects of the characters. For example, they could delve into why the characters react the way they do to the strange situations they're in. Maybe it's about their fear of change, or the way they've been indoctrinated by the 'lessons' they're being taught. The fanfic could also be a way to give the characters more agency, as in the original they often seem to be at the mercy of the surreal events.