Well, because I really enjoy my ad900x's. I love the SQ, especially when paired with the Fiio E12 (v1)'s bass boost. And they are very comfortable. Basically, I am not feeling any urge to jump off the ad900x bandwagon. And for the price they can be gotten for now, they are a no-brainer as far as I'm concerned.
I have read that the imaging is maybe not as good as with other headphones, and I can understand that. But it is plenty good enough for me. If they were going to be my only headphones, then maybe I would be obsessing about it more. But I am very happy with the ATH trifecta that I have going on - ad900x for open air goodness, ws99 for bass goodness, and CKS1000's for in ear goodness