Researchers want to get their work done. They don't want to fight against their tools.
There's also the issue that by relying on proprietary frameworks that work now, you might be painting yourself into a corner if Nvidia changes something in the future and then you have to adapt to them because you have no choice.
When nvidia sees a need, they can change CUDA over night to address it, and they pay people to do that.
When you need to do the same in Vulkan, that’s a multi year process till your extension is “open”. A teaser here whose job is to get something done with ML has better things to do than going through that.