As a Rust + C++ developer (in medical right now, and vehicle before that) I see no reason why I couldn't also pick up Ada as one of many skill sets. I have an Ada95 manual on my bookshelf from years ago that I bought out of curiosity -- frankly for senior level talent the language syntax is the easiest thing to pick up it's the intricacies of an existing code base and business domain that is the hard part of joining a new project and that is generally language independent.
Arguably picking up Rust -- with its frankly exotic value passing and ownership semantics -- is much harder than learning Ada.
> I see no reason why I couldn't also pick up Ada as one of many skill sets
Low wages and the negative resume drive development might be reasons.
Believe it or not, pigeon holing yourself in niche/uncool tech for years, can and will negatively impact your future hiring prospects at good jobs, since hiring in tech is broken and HR selects resumes on buzzwords and on your "last X years of experience in Y framework".
Arguably picking up Rust -- with its frankly exotic value passing and ownership semantics -- is much harder than learning Ada.