I'd say it's likely because NVIDA's CUDA compiler produces code for NVIDIA GPUs, and AMD would have a lot of (questionably legal) reverse engineering ahead of them in order to support the same code on their own GPUs.

If you're asking why AMD doesn't make a compiler for CUDA source code that targets their own GPUs--that's basically what ROCm currently does. They're pushing their CUDA alternative, called "HIP", which is essentially just CUDA code with a find-and-replace of "cuda" with "hip". (And other similarly minor changes.) They have an open source "hipify" program that does this automatically (https://github.com/ROCm-Developer-Tools/HIP/tree/master/hipi...).

So, basically, AMD GPUs are already sort of CUDA compatible: just run your CUDA code through hipify, then compile it using the HIP compiler, and run it on a ROCm-supported system (which, for now is the most spotty of all of these steps IMO).