Vulkan isn't supported on MacOS either. They've gone with "Metal" instead.
Vulkan will be portable to other OS's besides windows, but the developers will still need to do the work (hence doom only being windows).
Same thing with OpenGL vs DX we've seen before. OpenGL / Vulkan are both available more places, but DX offers more features and has better tooling (inc Visual Studio).
Both are great, but most developers won't use either for a while because they are much lower level and require lots of knowledge of how GPUs and graphics in general actually work, not just basic calls from high level apis. (Many devs don't even make direct DX calls but use a wrapper like Unity).