Nvidia is the default option because it's convenient at all levels of the stack. Close to two decades of investment into CUDA made sure that GPGPU with Nvidia is extremely smooth, and AMD support is a lot more spotty and less maintained. But add arbitrary restrictions like needing some outdated kernel or a special distro, and suddenly getting an AMD card to work with your favorite framework doesn't seem like so much more work. And once that reaches critical mass and patches are upstreamed, the convenience advantage of Nvidia is gone and competition will get a lot more fierce.