Unlike Linux, Windows and etc. I don't think even GPU makers can provide their own implementation of Vulkan there - only Apple can do that. And they refuse. So it can be considered a deliberate blocking.
I personally believe, considering their track record, it’s more likely they simply don’t care about it.
More experienced macOS and iOS developers will probably be able to confirm that Apple basically only invests efforts in what Apple wants, whatever that is, and nothing else.
