Why not Go? It's more portable.
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I'd imagine the biggest cultural reason is that many Rust developers were C developers who had a reason to find something better, but still scoff at garbage collection, large runtimes, etc. They probably have a lot more Rust expertise in their circle.
Another technical reason is that they were trying to replace their C code with Rust in bits and pieces before they went with a full rewrite. I don't know about Go, but this is something ergonomically doable in Rust.
I like loose type systems for some quick scripting, but I started to adopt Rust for many of my personal projects because I find it's so much easier to get back into a project after a year when there are good type system guard rails.