OpenGL ES only took off thanks to gaming on the iPhone, and now is deprecated on Apple platforms.
Vulkan still lives in a C world, and the semi-official C++ bindings only exist thanks NVidia.
OpenCL waited too long to support C++, Fortran and providing an infrastructure for compiler writers to add GPU support to their own languages. And two years later the majority of drivers are not there yet.
Which Khronos finally adopted as SPIR, but the drivers aren't there yet.
Regarding the other Khronos APIs, a C API is like being stuck in a PDP-11 world.
Many mix the idea of C API with OS ABI, it only happens to be the same if the OS APIs are exposed as plain C.
There many cases where this isn't like it, e.g. mainframes, mobile OSes, and most userspace on OS X (Obj-C runtime) and Windows (.NET and COM).
Yes driver support is a bit lacking, although I hop that I can convince the OpenCL working group of the need to get a backend (such as https://github.com/thewilsonator/llvm-target-spirv) into mainline LLVM so that writing drivers becomes easier for vendors.