This is the main reason. Also legacy. I work on a lot of embedded code for $AUTO_MANUFACTOR some of our code bases go back to the 90's.
The code base is so modified by macros/typedefs its hardly even C anymore.
Not to mention the LLVM has to support the embedded device you are targeting. And Rust's support of legacy CPU's (8008, 8080, 80386, 68000) are lacking.
