Fuchsia was started before Rust 1.0, and you still can't do OS dev on stable. It would have been cool, but I also don't think they inherently made the wrong choice.
That said, there's a few hundred thousands of lines of Rust in there, and more on the way. The Fuchsia team also lets some of its team contribute back to Rust development, which is far more support than having the kernel part of a microkernel be in the language, IMO. (You can see this in Futures and async/await, which Fuchsia makes heavy use of, and the team has been very helpful in driving the design and implementation to completion.)
