MMIX is created by Prof. Knuth purely for expressing his algorithms. Back then C++ and Java were the latest, hottest sh… (and would soon be outdated themselves). Prof. Knuth didn’t like C++ and general programming language fashion. So he invented MMIX. It was never meant to be implemented in real HW.
I don’t know whst’s wrong with the other CPU architectures you listed. I especially wonder why Sparc wasn’t used more.
I just stumbled over this project:
https://libre-soc.org/