Metal has the benefit of being developed by Apple for Apple devices. I'd imagine that constraint allows them to simplify code paths in a way Vulkan can't/won't. Again, Metal doesn't have to deal with supporting dozens of different hardware systems like Vulkan does.
Metal also works for external GPUs like NVIDIA or AMD though (not sure how much effort Apple still puts into those use cases, but Metal itself is flexible enough to deal with non-Apple GPUs).