The problem is that embedded systems today can range from a 1Mhz µC to a complete PC like a Raspberry Pie. While memory management is important for the latter, the former doesn't like any dynamically allocated memory at all.

Also many processor manufacturers have hundreds of different models with similar but different port mappings. They often supply C-header files which helps making your software compatible with numerous different processor types. So even while Rust has a macro system, it would need to be adapted by those manufacturers or C will likely stay the more convenient choice for the time being.