Because the presence of two sides in the story and their representation of light and darkness made the story remarkable. In addition, the budget and time that Riot allocates to the cinematics shot for the stories also drew attention to the stories.
The fact that each champion has a background story and the detailed processing of these stories is very interesting to fans. At the same time, the intersection of the stories of the champions and following which ones will intersect when, also drew the fans into the story.