I think that's mostly because there hasn't been any new mainstream language with the feature set and compilation model needed to port fundamental system components for a while. I remember lengthy debates about porting just about everything from C to C++ in the Linux world and even C++ was deemed unsuited for many reasons (with which I generally agree).

It turns out that Rust is compelling enough that it makes sense to investigate porting lots of things to it. I don't think anybody knows for sure exactly for which projects Rust is better than C (or C++), but I know that Google has ported many fundamental C++ projects to Rust and they're happy enough that they're continuing this trend.