As DirectX 12 and Vulkan are two APIs that accomplish the same thing with a few differences in their additional functions. Why not add Vulkan API support to the engine alongside the DirectX 12 support currently being added?
Intel, AMD, and Nvidia have shown demos of the Vulkan API running on Preview versions of the Vulkan API driver. All the demos show that Vulkan is a HUGE boost in performance and resource optimization over openGL. the problem with only supporting DirectX 12, is that if your user base wants to get the performance boost in the game, they HAVE to upgrade to Windows X. Many of us have no desire to upgrade and we like our current OS just fine.
Additionally, with the imminent release of the Linux and Mac OS beta client ports which AFAIK are running on a wrapper via Wine or something similar, it would drastically improve the experience on those platforms if Vulkan API support was added in instead of emulating DirectX. If the Linux/Mac ports were designed to only use Vulkan, that would give BI the chance to test the API without affecting their main player base. If everything works as expected or exceeds expectations, they can then add it to the Windows client so that all the users not running MS Windows X can gain the same performance benefits of those running DirectX 12 without having to upgrade.
Below you will find youtube links to the relevant Vulkan API demo videos.
https://www.youtube.com/watch?v=P_I8an8jXuM
https://www.youtube.com/watch?v=0Hth4u65zfc