They present on Windows because gpu (driver) vendors choose spend tons of their money to make them happen. And they spend their money exactly why? Because they see their profit in Windows pc market.
I would argue that Linux Vulkan support from Intel/AMD/NVidia is likely aimed more at Android than traditional Linux desktop distros.
Also, the whole point of Vulkan is to move these card-specific optimizations outside of the core driver while still letting games use kernel space to interface directly with the hardware (and doubtless platform vendors will put an abstraction layer on top of that like Metal on iOS). So the whole situation I'm describing has been a huge pain for a long time, but things are starting to get better.