Why doesn't Dc or Marvel use Avatar?

I love Avatar Airbender and getting to like legend of Korra but I am wondering why Dark Horse is the only one who gets this cause it definitely did very well on Tv and I can see it definitely turning into a comic hit if DC or Marvel gets the brand.So your thoughts

Bump

DC and Marvel both have characters that they own that are worth millions to them. Why would they bother paying license fees? Dark Horse's forte seems to be making licensed comics like Star Wars, so it makes sense on their part.