CIG has decided to drop DirectX 12 and go with Vulkan API, specifically because using DiractX12 would restrict their user base to only Windows 10
"Cloud Imperium Games' upcoming space simulation game Star Citizen is en route to discontinue support for DirectX 11 and switch to the Vulkan API, while also abandoning previous plans of supporting DirectX 12.
The Director of Graphics Engineering at Cloud Imperium Games, Ali Brown, revealed this piece of information on the official Star Citizen forums, going on to say that the Vulkan API's support for Windows 7, 8 and 10, as well as Linux, were the main factors in making this decision, as using DirectX 12 would restrict their user base to only Windows 10 users.
For those unaware, DirectX 12 and Vulkan are low-level graphics APIs that, if used correctly, can utilize the GPU much more effectively than previous iterations of DirectX and OpenGL. This leads to better frame rates and lower CPU usage in games.
However, Brown doesn't dismiss the possibility of DirectX 12 support for Star Citizen in the future. As he puts it, "DX12 would only be considered if we found it gave us a specific and substantial advantage over Vulkan". He also mentioned that the two APIs "really aren't that different".
Considering the similarities between the graphics APIs and the ongoing development of Star Citizen, Cloud Imperium Games could easily change their stance on the exclusivity to Vulkan. Although, as Vulkan already supports Windows 10, focusing development time on a single pipeline could be the optimal path to take, since the developers are probably hoping to avoid further delays to Star Citizen.
In fact, we probably won't have to wait much longer to find out if the developers will have a change of heart on this decision, as Star Citizen's single player campaign component, Squadron 42, is aiming for a release this year, at least according to the official website, barring any hold-ups that have plagued the game since its inception."
Main Source: https://forums.robertsspaceindustries.com/discussion/comment/7581676/#Comment_7581676