Nvidia GTX 9xxM - from what I understand 10xx cards offer the biggest difference in performance generation-to-generation seen in a while on the mobile side - the 10xx mobile versions are basically identical to desktop versions, 9xx are not even close - so why put last gen mobile tech in to a high end professional desktop product ? Especially considering the likelihood of VR proliferation in content creation - I couldn't justify buying this just because of that considering the price tag.
I imagine the protracted development time is at fault. The guy from Penny Arcade wrote a blog post where he said that he tried a prototype a year ago, so presumably this has been in development for at least a year and a half. Probably the 9xx was the best they had at the time and when the 10xx was released they were too far along in development to swap it out. The next one should have it though.
I was also wondering about that. They probably had to do a lot of customizations on the board to fit it in such a tiny place and cool it effectively. Designing such things can take quite some time. It is a shame, though