Vulkan is counter to Apple's vertical integration approach. It would be a foundational API, sitting between their graphics hardware and their OS, yet one they don't control.

Better for someone else to build a Vulkan API on top of Metal, which is what has happened. It's not perfect, but it's the only thing that can work. The pressure on Apple should be for Metal to better support Vulkan by providing APIs it needs to work optimally.

Beyond that, Apple might want to contribute to the Vulkan-on-Metal implementation... though they're only going to do that if it makes strategic sense, which I don't see. For cross-platform, what makes more sense is to encourage games to use a higher-level engine that supports metal among its platforms, like Unity and Unreal.