Can someone explain the technology edge Nvidia has?

AMD, Intel etc. have not been able to compute in high-performance GPU market, so Nvidia must have an edge. How big and sustainable it is?

They don't really have an edge today. They just achieved big lock-in, and inertia of those who depend on CUDA now prevents them from using other hardware.

What prevents other vendors from making CUDA compliant hardware? I thought the result of Oracle v. Google is that APIs can’t be copyrighted?

Even if you assume APIs aren't copyrightable, I don't think anyone cared to implement CUDA itself except for Nvidia. There is really no point in proliferating such APIs, since there are OpenCL / Vulkan already which are open to begin with.

What someone could try implementing though, is translating CUDA into OpenCL (if that's possible). That would be useful to break lock-in.