Also, Vulkan is quickly becoming the lingua franca of graphics APIs. Vulkan can be implemented on top of Metal, which means that developers have next to no reason to implement Metal directly.
Actually, it's Valve paying the bills for the MoltenVK guy so that MoltenVK is part of Khronos.
Basically, Valve is saying: "OS X is a completely useless environment but has lots of people who spend more money than Windows users. So, we can make lots of money if we can maintain one codebase by writing a shim wrapper even if we have to eat performance."
Performance could be a reason. It’s highly likely that implementing a low level graphics API on top of another low-level graphics API will have some performance gotchas here and there.
