That's not the actual reason. Hexbear was openly advocating for their "army" to brigade other instances once it was federating. It just so happens that the basis of that brigading was going to be political.
Lemmy.world pre-emptivly decided it wasn't worth the hassle of having to deal with that.
Compiled Rust is fast.
Compiling Rust is slow.
Also my understanding is that RustAnalyzer has to compile all Rust macros so it can check them properly. That's not something that a lot of static analysis tools do for things like C++ templates