The fanbase likes to explore different relationships within the game's universe. Soldier 76 and D.Va are both popular characters, so seeing them interact in fan - created stories allows for a deeper exploration of their personalities and how they might react to each other in various situations. There could be a story where D.Va's enthusiasm rubs off on Soldier 76 and makes him see things from a different perspective, or vice versa. This kind of character exchange is very appealing to readers of fanfiction.
The contrast between their characters. Soldier 76 is this old - school military veteran, while D.Va is a young, tech - savvy gamer - turned - hero. Their interactions can lead to really interesting character development.
It's the potential for unique storylines. For example, they could be forced to work together in a situation where their skills are both needed but they initially don't get along. This creates a lot of room for conflict and resolution in the fanfiction.