Well, IMHO, "the right hammer for the right nail". Rust is a great systems language (besides the lack of bitfields) but I wouldn't use it as a web language. Just as much as I wouldn't use C++ to develop webapps. Sure you can do it but then again there are far easier ways to achieve your goal.
Out of curiosity (I've never done systems programming), what are some of the things that make a language more suited to systems programming rather than web (other than libraries and community support - I'm more interested in the intrinsic qualities of the language)?
Ability to convert between language types and raw addresses, in the form of pointers.
Toolchain should also provide the ability for aot compilation both to static and dynamic executables/libraries.