It was never really a competitor, as the other two sponsors Intel and AMD, never deliverd anything great with it.

Additionally the tooling is horrendous, plain old C, with the same compilation model as OpenGL.

It took getting a hard beating from CUDA, to finally add a bytecode format (SPIR), and at least support C++ as well.

Additionally the other mobile OS big name never cared about OpenCL, rather pushed their own thing, Renderscript.

So, to reiterate: Apple used to have a CUDA competitor that was so bad you guys get mad when I call it competition.

And after Apple dropped NVIDIA, they stopped caring about openCL performance on their GPUs.