It just was a completely different era. There was no consept of what we today call open source, no compilers freely available over the internet, no internet to speak of for that matter, no cross-platform/multi-platform compilers to speak of, more platform diversity than now, FORTRAN was also a vastly smaller language so you could have implemented a compiler in a reasonable time if you didn't want to shell out $BIGBUCKS to buy one, optimizers (which are deep magic and really hard to write from scratch) weren't yet a thing, there were no standardization bodies that worked on programming languages, everything was vastly less connected than today. That obviously leads to redundant work.

No matter, the point is that nothing has ever stopped anyone from writing a competing Rust compiler, certainly not any imaginary "committee." Nothing except the fact that, besides mrustc, nobody has bothered because why would you do that?