Direct x already is the default api. Vulcan is used but it’s like windows vs Mac os usage numbers.
Directx has to be created first as the three companies almost co-design it nowadays. Nvidia and Amd have heavy inputs in Directx and then they follow up by keeping Vulkan up to date as well.
On the PC side it is not enough to say, we’ll do with only Vulkan. Although it is a good low level api, Directx has a 20 year legacy and uis usually defacto. Sony’s gnm and gnmx apis are only for Playstation hardware. They will have to move to Vulkan/Directx anyway.
That is why the Directx 12U is such a game changer. One set of low level apis for console and PC.