It's used by those because it's pretty much the only option available right now. If OpenCL managed to produce industry strength programs with either NV tesla or AMD's firething (can't remember the name) I'm sure we would see further advances in research etc. Seen as how NV want to monopolise their CUDA program and keep it as a closed platform it's a shame really but any company would do that if they managed to write a terrific C++ prog.
sigh...