There is an entire document which explains why they can't use rust or zig.
The short version is that for their particular usecase they need really close ties to C++.
"Existing modern languages already provide an excellent developer experience: Go, Swift, Kotlin, Rust, and many more. Developers that can use one of these existing languages should. Unfortunately, the designs of these languages present significant barriers to adoption and migration from C++. These barriers range from changes in the idiomatic design of software to performance overhead."
I guess I'll rewrite my original comment here to clarify
I don't know why google created this language. It offers nothing new and I can't see why they'd use this over rust when rust can be used. I don't see why they use this over zig since zig can certainly be used where this language is meant to be used. Zig actually brings something new
Overall google is 0/3 on creating languages that people want to use. Maybe go is useful but I haven't seen enough proof
The short version is that for their particular usecase they need really close ties to C++.
"Existing modern languages already provide an excellent developer experience: Go, Swift, Kotlin, Rust, and many more. Developers that can use one of these existing languages should. Unfortunately, the designs of these languages present significant barriers to adoption and migration from C++. These barriers range from changes in the idiomatic design of software to performance overhead."