Arthur D. Hlavaty

Maybe you can't just assert it, but I sure can. Of the many reasons why people write fanfic, changing representation is right up there. As the next commenter replied, ST:TOS had so many possibilities that it didn't show. That show's fanfic, including slash, birthed the current movement.

Also, isn't that why it's called the Organization for Transformative Works? Not the only transformation, for sure, but gender representation remains important.