Well, it's appealing because of the character interactions. Think about Naruto, a character full of energy and a strong sense of justice, interacting with Sebastian from Kuroshitsuji, who is so sophisticated and has his own dark motives. The contrast between the values and personalities of the characters from both series can lead to some really engaging storylines. Also, the fanfiction allows for new interpretations of the existing storylines of both Naruto and Kuroshitsuji, expanding on their worlds in unexpected ways.
The appeal lies in the potential for new and unique plotlines. Since both Naruto and Kuroshitsuji have their own rich mythologies and character rosters, fanfiction writers can create all sorts of scenarios. For instance, they can explore how the power systems of the two worlds interact. Or they can have characters from one world learn and adapt to the culture of the other. It's like opening a door to a whole new world of possibilities where the best elements of both Naruto and Kuroshitsuji can be combined and explored.