Well mainly because Nvidia gimped their OpenCL drivers and there was no customer pushback.

OpenCL vendors never provided a competing tooling alternative to CUDA.

Drivers are only part of the story.

Even Google preferred to create their own Renderscript dialect than supporting OpenCL.

That would still be fine if OpenCL was similarly-performing on Intel/AMD and a 1st class citizen of various Deep Learning frameworks. OpenCL is just an afterthought there sadly.