Vulkan doesn't do C++ as shading language for example, there are some backend attempts to target SPIR-V, but it still is early days and nowhere close of having the IDE integration, graphical debugging tools and rendering libraries that CUDA enjoys.

Examples of rendering solutions using CUDA,

https://www.nvidia.com/en-us/design-visualization/solutions/...

https://home.otoy.com/render/octane-render/

It is definitely ergonomics and tooling.