Потому что сам Google в инфе о карбоне пишет, что если может вместо Carbon использовать Rust - используйте Rust

https://github.com/carbon-language/carbon-lang/blob/trunk/do...

> If you can use Rust, ignore Carbon
> If you want to use Rust, and it is technically and economically viable for your project, you should use Rust. In fact, if you can use Rust or any other established programming language, you should. Carbon is for organizations and projects that heavily depend on C++; for example, projects that have a lot of C++ code or use many third-party C++ libraries.
> We believe that Rust is an excellent choice for writing software within the pure Rust ecosystem. Software written in Rust has properties that neither C++ nor Carbon have. When you need to call other languages from Rust, RPCs are a good option. Rust is also good for using APIs implemented in a different language in-process, when the cost of maintaining the FFI boundary is reasonable.
> When the foreign language API is large, constantly changes, uses advanced C++ features, or makes architectural choices that are incompatible with safe Rust, maintaining a C++/Rust FFI may not be economically viable today (but it is an area of active research: cxx, autocxx, Crubit).