As this article indicates, it may just be a strategic marketing move on BOTH GPU manufacturers to wait for the other one to drop their next-gen GPU - if they drop too soon, they could be shooting themselves in the foot, so to speak.
Yeah I don't really see any reason to hurry. From what I can tell, AMD is promising a product that is essentially a 1080 Ti but with some features which Nvidia decided to just not bother consumers with and instead push them for Volta (but Pascal workstation cards have them though, looking at you Unified Memory vs. High-Bandwidth Cache). I honestly they did it out of greed and because they didn't perceive AMD as a big enough threat... until suddenly you start seeing GP102 chips flooding the market without being neutered, at a price you supposedly just can't refuse. AMD has only one chance to do this right, better safe than sorry anyway. It might be the comeback of the decade or the biggest disappointment of the century. If there's any room for something between those two options, I think it's going to be pretty narrow.
I'm not particularly on either side of the fence, I admire both companies currently for very different reasons though. I have to say though, it's been way too long since I last owned an AM.. ATI card. It's been way too long since I updated my gpu too, but hey, it's not my fault Nvidia can't (won't, really) offer a reasonable upgrade for 780 Ti which has gotten me this far reasonably well.