Only with OpenCL 2.x have they came around and started to support C++ for writing kernels, as well as, a standard bytecode for any other language to target. Which most vendors still don't support.

Whereas CUDA supported C++ and Fortran from day 1, with the PTX support added a few versions later.

Also the debugging tools, from the presentations I have seen, are much more developer friendly on CUDA.

Of course developers rather use APIs that offer more modern experiences than ones still stuck in pure C, with a compiler at the driver level, forcing each programmer to writer the boilerplate to compile and link.

Now it might already be too late for OpenCL in spite of the latest improvements.