Apple was the company that boosted OpenGL popularity by using it as the graphics API for OS X. In Apple platforms world there's only one graphics API that is guaranteed to work across their hardware, so it was a big deal. However, they could not keep up with OpenGL updates, as they overlapped poorly with their products map, i.e. switching new OS X to newer OpenGL revision would require to drop support for older Macs.
Out of that need (and also to address multiple shortcomings of OpenGL) Metal was born. OpenGL support layer was implemented on Metal.
Metal was released before Vulkan API was finalised. There was never need for Apple to support Vulkan. Vulkan, as the OpenGL before it, has the same downsides for Apple, but bringing nothing to the table compared to Metal.
Very few games have custom written Vulkan rendering pipeline. Majority rely on game engines, and if the engine supports Vulkan rendering, it is almost certain to also supports Metal.
So, instead of relying on supporting 3rd party rendering APIs Apple spends resources on helping with porting games to Macs natively.