I think the answer is probably that Rust was possible in the 1980s and 1990s, but such a thing just wasn't practical.
Rust is notoriously compiler-intensive. That wouldn't have been tolerated in the early PC era. When you needed fast compilers that "worked on my machine" and should work on yours. Ship it.
It wasn't really possible. We had neither the PL techniques nor the computational power to make something like Rust work at the time. All the answers people are throwing around showing it would have been possible rely on a garbage collector and require a runtime, or have many other unacceptable compromises (e.g. no use after free because you aren't allowed to free).
