V8's API is C++. While bindings exist in other languages, they don't always expose everything and don't always keep up with V8's frequent API changes. I felt it was essential that we bind closely to V8, being able to use all of its features, and that we be able to track closely to the latest stable release.
Also, I personally have decades of experience in C++ and wasn't sure this should be the project where I work on learning Rust.
That said, I love Rust and highly encourage more people to use it.