Metal release date: 2014
Vulkan release date: 2016 (announced 2015)
So since somebody made a non-compatible clone a couple of years later, Apple should have ditched their work and lost the power to make decisions as they see fit in the process? That doesn't sound like something that would appeal to them.
Unfortunately, the 2014 Metal release is not comparable to 2016 Vulkan release.
Metal from 2014 had a lot of assumptions (for example, that the GPU shares memory with CPU), so it could be released earlier due to being simpler. That's why it was supported only on iOS.
And that's why there were more Metal releases and refinements. Apple had to make improvements to make it equal in its capabilities to Vulkan 1.0 and DX12.
So since somebody made a non-compatible clone a couple of years later, Apple should have ditched their work and lost the power to make decisions as they see fit in the process? That doesn't sound like something that would appeal to them.