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.
You know Vulkan often changes or update their spec? It's not a nonsense for Apple to develop their own with easier API also made for Swift language, reliability is important.
An AV codec isn't a weapon to lock developers into your platform while wasting millions of dollars and thousands of man hours on pointless api ports. There are too many implementations of encoders / decoders for the various formats lying around to get away with that. Its not like they didn't try with Quicktime in the past (ish? .mov was just a crippled .mp4), but that never really made them any money I don't think.
They invested heavily in their own API (Metal) while OpenGL committee was sitting on its ass doing nothing, and released it a year before Vulkan was even announced.
