Earlier quoted context omitted.
Metal is a proprietary 3D technology ported from iOS. Apple chose to do another closed tech instead of adopting Vulkan or fixing their slow OpenGL implementation. If you look at the numbers, you'll see that Metal is about 30% faster than OpenGL on OS X, which is interesting because OpenGL is about 30% slower on OS X than on Windows. Which means they're forcing developers to use their proprietary API to get comparable…
To be fair (regarding "closed tech"), Apple was hardly the only one to release a propriety Vulkan alternative (there's also Mantle and DX12) and Apple released Metal to developers almost two years before Vulkan 1.0 was released.
In contrast Apple exclusively controls the macOS graphics stack top to bottom, so as long as they don't explicitly support Vulkan they're forcing developers to use their proprietary API.