CUDA was there first and has established itself as the standard for GPGPU ("general purpose GPU" aka calculating non-graphics stuff on a graphics card). There are many software packages out there that only support CUDA, especially in the lucrative high-performance computing market.

Most software vendors have no intention of supporting more than one API since CUDA works and the market isn't competitive enough for someone to need to distinguish themselves though better API support.

Thus Nvidia have a lock on a market that regularly needs to buy expensive high-margin hardware and they don't want to share. So they made up a rule that nobody else is allowed to write out use something that makes CUDA software work with non-Nvidia GPUs.

That's anticompetitive but it remains to be seen if it's anticompetitive enough for the EU to step in.