To put it even more plainly than the others: https://github.com/carbon-language/carbon-lang#project-statu...

> Carbon Language is currently an experimental project. There is no working compiler or toolchain. You can see the demo interpreter for Carbon on compiler-explorer.com.

Without trying to sound dismissive, Rust is production ready today, Carbon isn't. Even if Carbon was significantly better, that alone accounts for the adoption of Rust and not Carbon today.

Carbon is sort of a plan B, for working on existing C++ code that would be too difficult to migrate to Rust. It also doesn't really exist yet.

That's more of a moonshot strategy. Rust is more of a safer bet.

Google is a very big company which has many parts that aren't all necessarily aligned.