Probably because Smith (1 of the 3 tools) is written in Go and because Oracle wouldn't use Rust just for the sake of using Rust but because it was suited to the task at hand.
edit: you might be interested in their article on "Building a Container Runtime in Rust" [1]
With that we have now Microsoft and Oracle adopting Rust based tools.
Even if Rust fails to gain wide adoption, their ideas already tainted (in a very positive way) future work on the design of Swift, Pony, D and C++ lifetime static analyzers.
Also on Go side, Microsoft being a Docker contributor regarding its support on Azure and VS Code tooling.
Which I find very positive for both communities as a language geek.
