There are Kronos extensions that require Win32 (and many of those do not have other platform equivalent from dealing with unified memory, to DX11/12 integration and other features), Vulkans mGPU support in it's current form specifically mandates Windows 10 because it's based on how WDDM and Windows 10 allow you to link devices, so do some of the VR specific functions.

Vulkan was a Camel before they introduces the new extensions road map, you know have KHR which are "official" cross vendor LTS extensions which can be still platform dependant (e.g. you can have win32 KHR extensions), then there are KHX extensions which are effectively a rapid deployment branch primarily sucking in vendor extensions that can be a good idea, then there are the vendor specific extensions.

Now not only that it's a camel, it seems to fall back into the same tarpit that OpenGL had in which you could not guarantee compatibility between hardware vendors and platforms.