Muse talked about DX12 in a Fireside last year. Maybe more than once. Basically comes down to compatibility and if new Unity updates don't bring a slew of new bugs with it. They do test them and if it breaks more things than fixes, Muse doesn't do it. Pretty iffy we'll see DX12 features super fast.
Besides that they just last year added a DX11 mode for the game. Not sure what that all entails but we'd more likely see extensions on that before DX12. Also some of the feature sets in DX12 are the same ones that were implemented in DX11.1 and 11.2. Which no one really supported because no one bought Windows 8. Hence why ASYNC compute now starts to matter because Windows 10 adoption is much greater.
On top of all that, even the studios working on DX12 games right now state the difficulties in changing over their development to it. ARK devs have continued to push back the DX12 version of ARK, citing all sorts of issues.
DX12 isn't some simple switch to turn on and enable amazing performance like a lot of the hype stated it would be.