They aren't releasing a new top end card because they don't need to, there is nothing that can come close to the 1080 Ti unfortunately, why release a flagship killer when there is no competition? It's been this way for ages.
This, plus the fact that half the 10xx series cards are still OP for a lot of the market. If Nvidia is smart, they won't do another major release until they have a true 4K card (4k/60FPS/ultra). Until that happens, any significant upgrade to their product lineup would be paper gains.
If they were really smart they would launch the new line of cards with a new brand of SLI based off of NVlink, eliminating software dependencies.