Why should they?

Other platforms don't have a leader that hates C++, and then accepts a language that is also quite complex, even has two macro systems of Lisp like wizardy, see Serde.

OSes have been being written with C++ on the kernel, since the late 1990's, and AI is being powered by hardware (CUDA) that was designed specifically to accomodate C++ memory model.

Also Rust compiler depends on a compiler framework written in C++, without it there is no Rust compiler, and apparently they are in no hurry to bootstrap it.