oneAPI is effectively an Intel-only platform not a standard.

Yes they have implementations on top of CUDA but they're maintained by... Intel. They didn't get buy-in for cross-vendor collaboration

They were maintained by Codeplay - paid for my Intel. Nvidia can make contributions anytime they want, and here is the problem: Nvidia does not want to. Until each vendor starts pitching in with contributing their backend to an open standard, you will have to rely on others doing it for them.