It's fair to have an opinion, and that may be why Kubernetes hasn't been rewritten in Rust, but I answered the question of why it wasn't written in Rust. It wasn't an option at the time (again, time travel is not possible).
This is a myth pushed by the "Choose Boring Technology" cult [1] and proven false by Google [2].
K8s was absolutely built using exciting tech....Golang was hot off the presses. The brand new hotness. Google's internal Borg was C++, so the boring choice was C++ which wasn't chosen. Also they were google employees, which also adds a bit of bias in selection I would imagine.