Literally all of these are owned by big corporations (Apple, Microsoft, Sony) and made specifically for their proprietary gpu APIs. C++ is just a superset of C which doesn't fix syntax problems. Making an open source standard with a modern syntax is a better idea than picking a favorite corporation.

Guess what is the main shading language used in Vulkan, specially going forward as announced at Vulkanised 2023.

C++ is a safer, more expressive superset of C89.

Also those dialects aren't C++, they are based in C++, which isn't the same.