CUDA had a lot of inertia and opencl brought half baked docs and half baked support out of the gate. If they had focused on simplifying their api to be more user friendly for the 80% use case it could've been a success. Opencl always looked nice on the surface but a few hours in and you've exhausted the docs trying to figure out what to do and there's no good example code around. Of course if they really wanted it to succeed they would've built a Cuda to opencl transpiler for the c api or at least a comprehensive migration guide. I'm not convinced anyone involved was trying to make it popular.