For similar reasons why Microsoft created DirectX. It allows them to have a system where the software and hardware are more tightly integrated than using a cross-platform spec. It also allows them to situate the API within the context of the operating system and other languages that are used on Apple platforms, making things easier on developers. And at least in that regard, they certainly succeeded. Metal is probably the easiest GPU API offered on any platform. Not necessarily the most powerful, but it’s almost trivial to hand write a compute kernel and spin it off.
I don't understand. If OpenCL was so good, why did Apple create Metal instead of just using OpenCL?