Because these applications were created long before Rust existed.
There is a cost to conversion.
Rust-Brotli (by Dropbox) is actually a drop-in replacement for the Brotli C lib: https://github.com/dropbox/rust-brotli#:~:text=rust-brotli%2...
Dropbox switched to it a decade ago, so it's probably quite product-ready.
Brotil was actually created post rust, but before rust took off.
Check Librsvg, or php-rust. Or tons of gtk4-rs software doing it just fine.
It's not even an application it's a compression library