With the exception of Onyxia(and that is kinda cheating because she was the only raid boss) The Eternal Palace is the first raid where the number of female bosses is greater than the male bosses. Add to that it was just something that sort of happened. It was not some feature/or selling point or whatever, it just happened to make quite a lot of sense for the raid in question.
Obviously considering Azshara was the final boss it was always expected. Personally, I think WoW does need more female villains. I like the idea that both men and women can be equally evil.