Why should they support vulkan? what does apple get out of that apart from less well optimised compute and shader code, using more battery and producing more heat for the same output. (the reason it would be less well optimised is Vulkan is an API designed by a group to be the best compromise of many GPU vendors.

If apple wanted to support vulkan without it being worse than Metal they would either need to add so many apple only extensions that it would be Vulkan in name only or make their GPUs be identical to AMD or Nvidia (unfortunately due to IP patents apple can't just make a copy of AMDs GPUs they need to find another IP partner and that is PowerVR).

If PowerVR had 80% of the GPU market (like Nvidia) the would have pushed Vulkan to line up with a TBDR pipeline but they do not so while you can run Vulkan on a TBDR pipeline you end up throwing away lots and lots of optimisations.