Because except in rare cases Rust can do everything C++ can do with basically the same performance profile, but it does it with modern tooling and without the security, reliability and productivity issues associated with C++'s pervasive Undefined Behaviour.

There are some cases where C++ makes sense:

* You have a large existing C++ codebase you need to talk to via a large API surface (C++/Rust FFI is not great)

* You have a C++ library that's core to your project and doesn't have a good Rust alternative (i.e. Qt)

* You don't like learning (and are therefore in completely the wrong industry!)