I think it's because fans love to imagine how the characters from these two very different universes would interact. It's a way to bring together two popular franchises and create something fresh and exciting for fellow fans to enjoy.
Some reasons might be that fans see potential crossovers in the characters or themes. Maybe they think the magical world of Harry Potter could add an interesting twist to the serious issues explored in 13 Reasons Why. Also, it gives them a creative outlet to explore uncharted territory.
It could be because fans are drawn to the unique elements of both and want to create a new story that combines their favorite aspects.