Well if you remember..in, I believe it was, season four..Ahsoka has a vision of her future self...who warns her that she might never see her future if she continues to stay by Anakin's side. So I think its safe to say that the creators and writers of the show have something in mind to explain why there is no mention of her in ROTS.
They had better find some way to end the tale of Ahsoka. No matter what that ending is. So long as it makes sense when it comes to ROTS.