You have to keep in mind Nvidia only has so many engineers and R&D money for work on architectures. Nvidia inserted Erista in between Parker and Logan because they knew TSMC wasn't going to have finfets ready. They probably brought the Denver core up in release with schedule with Logan (aka Tegra 5) to A) get the first Google x64 tablet onto the market and B) learn where and what needs improvement on the Denver core for it's next iteration. Given all that, it's very plausible that they are still heavily working on tweaking / improving Denver and choose to spend their resources entirely on Denver's finfet design rather than porting to 20nm then porting it to finfets. I believe that Parker will have a Denver variant, if not be available only with Denver cores.
Anyways, Erista looks great. Here's hoping it's in more tablets stateside than TK1 was in. Nexus 9 would have been a buy for me if the storage options weren't so ridiculously limited.