OpenGL was a technological dead-end and Vulkan came 2 years after Metal, which as stated, came about because OpenGL was a technological dead-end. Besides, mobile gaming with Metal has been reasonably successful, whichever way you slice it.
I know Apple isn't exactly a company that's open to adopting other people's standard when they might as well develop their own, but the split from the other API standards was still a choice they made.