Same reason why NV released a 780 after the original Titan. Opened up a new market of buyers. People will see the ~$300 savings as a bonus since it will match Titan X speeds w/o the premium price for 12GB of RAM.
780 was well slower than Titan. It needed a good OC to match it.
The rumor here is 980ti class is a full GM200 with 6gb vram. That already frees up TDP to allow a higher boost out of the box. It will be faster than Titan X if it isn't a cut down GM200.
It's more similar to if NV launched Titan, then follow up a few months later with the 780ti.
Even then, Titan has the compute advantage & 3 vs 6gb vram to hold its price in place. Titan X does not. It's only got performance and 12GB vram as the advantage. Once it loses in performance, 6gb vs 12gb isn't much of a selling factor for nearly every single gamer (unless they are packing 4K & like to run with 8x AA or multiple 4K monitors), as 6gb is the "console port sweetspot".
It actually makes little sense to stomp over Titan X with a cheaper 980ti that beats it, this early without competition.