It is not only performance.<p>NVidia was clever to see that no one wants to code GPGPU code in C, rather leverage their C++ source code and the Fortran libraries with decades of investment.<p>So CUDA has been language agnostic from the first day.<p>Khronos has realized their error by creating SPIR, but it might be too late already.<p>Also the debugging experience for CUDA has always been quite good.