Rust is the best programming language, so the question is more why you wouldn't use it.

And the answer is you wouldn't use it if there's a library or existing codebase in another language that is much better than the Rust equivalent, and the program is short enough that it's simpler to directly use that language rather than writing a binding or RPC interface.

Or if you need the software to be written by someone who doesn't know Rust and doesn't want to learn (but in that case, they might not be able to write a correct program in any language, so this strategy is probably best reserved when a program that only works properly sometimes is acceptable).