You understimate how hard cross-platform is to achieve. Especially with GPUs. There is no GPGPU API standard, first and foremost, so ensuring cross-platform is a tedious task which essentially either means creating an API that has to accomodate every GPU, or writing "drivers" for every different GPU. GPUs can be vastly different between generations and models, unlike x86 and x86-64 CPU architectures which have mostly stayed the same for several decades now.

Vulkan Compute is nowhere near reaching feature parity with CUDA and cuDNN. ROCm's CUDA is way better and still too much of a pain to install and keep.

Furthermore, open standards mean nothing when a graphics vendor can just gimp the API, like, ironically, nvidia already does with Vulkan.

There is an open variant called OpenCL. But it will probably never be as mature as CUDA, even though 3.0 is apparently making great strides. There is absolutely no reason to push for Vulkan due to how cancerous developing anything in it is.