I'd imagine it's as simple as Nvidia doesn't want to support OpenCL because CUDA forces people into their market, and AMD doesn't want to support OpenCL because Rocm forces people into their market. Open/free standards are great for smaller players in a market, but in the graphics space I don't think a smaller player in the market exists, and if they do, no one seems interested in them.
I'd imagine it's as simple as Nvidia doesn't want to support OpenCL because CUDA forces people into their market, and AMD doesn't want to support OpenCL because Rocm forces people into their market. Open/free standards are great for smaller players in a market, but in the graphics space I don't think a smaller player in the market exists, and if they do, no one seems interested in them.