One major point of heartburn with Rust is that it comparatively lacks the diversity of ISA targets that C broadly does. I know some of this is because C is both relatively simple to write a basic compiler for that more or less just works (in comparison to something crazy like C++), and that's it's been around for a long time, but why isn't there more of a push to add at least all of the supported Debian ISAs to the Rust compiler?
Most people don't write a basic compiler for C either, "relatively simple" or no. Most people would rather add a new target to an existing compiler, which is much easier.
It's also "relatively easy" to add a new backend to Rust.
There's a policy document for Rust here: https://doc.rust-lang.org/rustc/target-tier-policy.html
There are a lot of things that can go wrong. You want to be able to test. Being able to test requires that someone has test hardware.
There's no push to add Debian's officially supported platforms to Rust because Rust already supports those platforms.