Remeber that until AT&T was allowed to take commercial advantage of UNIX, its source code was available for a symbolic price, and it came with a C compiler toolchain, at least until Sun started the trend among UNIX vendors of spliting UNIX development tools into an additional purchase.
The UNIX vendors that had Ada compilers, that was an additional purchase on top of UNIX SDK (which already had C and C++), so unless there was a hard requirement to use Ada, no one bothered to pay extra.
And it's not like it has no effect on the final third either. The stronger type system and ownership system make non-memory related bugs less likely too.
Your comment makes it sound as if you have a better solution. But Rust is the best one we know so far, so this kind of naysaying does more harm than good.
Now as GP was saying, Rust inherently mitigates a great deal of generally serious software vulnerabilities, which is great, even if it doesn't mitigate anything else.
The problem is that Rust solves _a_ class of issues without even beginning to address other kinds of problems, and so this sort of thinking is evolving toward a mindset that "oh it's rust so no security issues", which is completely wrong.