LGBTQ+

I think part of the reason is that Harry Potter is one of the more interactive Fandoms. The world in Ender's Game is more or less closed. The story is finished, the world isn't too big.

Harry Potter on the other hand (much like e.g. Star Trek) is way more inviting to be interactive: Which house are you, what would be your favorite class, what might the other schools look like etc. It is (by design or not) built to invite engagement and also very marketable. The special foods, the shops, all the gimicky in-world-items, the classes...

You get people so invested into this world beyond the books because there are so many details that are marketable. Other fantasy or SciFi worlds just aren't full of fun little items you can sell. In that regard Harry Potter is more like those kids shows that are made to sell toy lines.

So most people are way more emotionally attached to the stories. It wasn't just the books and a Halloween costume. It was years of choosing your house, learning about potions, discussing brooms, trying to cook the food, etc.