At this point, it's almost nonsensical to me that anyone would support DX12 at this point. Maybe MS's support and documentation is so much better than devs don't want to deal with it, but it makes no sense not to use an API that offers nearly an identical feature set as DX12, is open source, and it's cross platform, which lowers development costs.
On a side note, since Star Citizen is a game that's meant to be supported an upgraded for years, would that mean that the team will introduce new visual enhancements even after the game's release? It would be cool to see a game that visually improved as hardware got better. I guess the real incentive for this would be if they had a monetary platform to support such visual upgrades...