Rust only recently hit 1.0, so it's pretty obvious why somebody would not have built their software using that.
Also, current benchmarks show Rust to be about ~3x slower than C, making it more comparable to Go or Java.
Interesting. Do you know of any good, recent benchmarks of Rust? I haven't seen a whole lot. The Benchmarks Game mostly appears to show Rust being closer than that in general, though the Rust benchmarks seem to be kind of crap.
A few months back, we were beating C on some benchmarks, though with the focus on semantics for 1.0, I'm sure there were some regressions.