Metal was done [1] even before the Khronos Group issued a call to develop a next-gen low-overhead API [2] which became Vulkan. It helped that AMD basically donated Mantle so they were able to turn around in a year. Apple understandably was in no appetite to use DX12, and then-AMD-specific Mantle could not be construed as any more than a tech demo and/or an attempt to eke out more performance from their cards.

[1] http://www.anandtech.com/show/8116/some-thoughts-on-apples-m...

[2] (slide 20) https://www.khronos.org/assets/uploads/developers/library/20...