but I was surprised by how high the percentage of omegaverse fic was for BTS. I wonder why it's so popular there?

Although omegaverse sprouted in Supernatural fandom EXO fandom popularised omegaverse in non-western fandom after they released 'Wolf'. I suppose it made its way into other k-pop RPF from there, and BTS being the most popular k-pop fandom, one thing led to another?