Because OpenGL is a dead end for them and will make their engine less competitive in comparison with others. It's pretty obvious to me. Not sure why you think they'd rather use OpenGL than Vullkan if they have a choice.
Well, it's not like Vulkan is a "better OpenGL". Vulkan is much lower level than OpenGL. It does allow for better performances, being closer to hardware and giving more control to the developers, but it also puts a significantly higher burden on the developers, because it is lower level.
That said, The Witcher 3 has a DX12 backend it seems, and DX12 is somewhat comparable to Vulkan in the way it allows lower level access to the GPU (compared to DX11/OGL), so perhaps porting their DX12 back-end to Vulkan is not much harder (and would lead better performances) than porting their DX11 back-end to OpenGL.