Another reason is that fanfics allow for more creative freedom. Writers can take the basic elements of Edward and Jacob's characters and create unique love stories. They can break the boundaries set in the original story. In these fanfics, we might see Edward and Jacob in different settings or situations that force them to confront their feelings for each other. This gives fans a fresh perspective on these beloved characters and their potential relationships.
The popularity also stems from the fact that the 'Twilight' fandom is huge. Fans love to explore different relationship dynamics within the universe they are so passionate about. Edward and Jacob are two very strong and complex characters. Their love in fanfics can be a way to show a different side of them, a side that wasn't fully explored in the original books or movies. For example, how they could support and love each other despite their differences in species and beliefs.