Traditionally, government people have trust issues with programming languages as the compiler is, itself, an attack vector. If you are using a nightly release of the compiler, it may be assumed by some that the compiler is not vetted for security and could inject unstable or malicious code into another critical codebase. Also, Rust is considered very young for security type work, people rightly assume there are unfound weaknesses due to the newness of the language and related libraries.

Nevertheless, the government has allowed people to use Java for decades for highly secure codebases and it has had all kinds of issues.