To answer your second question directly: CUDA predates Vulkan. So if your question then becomes: Why did we need Vulkan if we have CUDA? OpenGL, DirectX, Metal

To answer your second question directly: CUDA predates Vulkan. So if your question then becomes: Why did we need Vulkan if we have CUDA? OpenGL, DirectX, Metal, and Vulkan all have lots of graphics-specific concepts built in to their APIs. While you could probably implement most of this in CUDA, at some level you need hardware support for your graphics APIs because in the end the data in your framebuffer goes from the GPU's memory to your monitor via HDMI or DisplayPort. CUDA has no interface for that graphics-specific stuff (technically there are CUDA-OpenGL interop APIs inside CUDA) because it has to be able to run on AI acceleration hardware in datacenters (although that hardware didn't exist when CUDA was invented either, to be fair)