Same reason Android and Chrome and git and Linux weren't written in Rust when they started. Rust didn't exist. All of these projects integrate Rust now, after being single language projects for the longest time.
It's notable that the projects you mentioned mostly don't need to deal with adversarial user input, while the projects I mentioned do. That's one area that Rust shines in.
It's notable that the projects you mentioned mostly don't need to deal with adversarial user input, while the projects I mentioned do. That's one area that Rust shines in.