It's funny how frequent people bring this up, but the truth is simple, check here [1]. Zircon kernel is not new [2], it has been in development for a while now. By the time the started to work on the microkernel, Rust 1.0 was really new, so they would've to implement several things from ground up. There's a implementation of Zircon in rust called zCore [3], but I don't know how stable and feature complete this one is.
[1] https://twitter.com/cpugoogle/status/1397265884251525122
[2] Like a two years project.
[3] https://github.com/rcore-os/zCore