Not easily portable to most microcontroller targets: No SDK for our target SoCs and official SoC/µC support. Increased code size (which is also a performance issue in many SoCs). High cost of retraining engineers. High cost of reenginering existing code base and tooling. Lack of commercial support.

I'm optimistic about Rust, and it taken great strides to replace C, but there are still many hurdles that prevent it replacing C in microcontrollers and SoCs. Even if it wasn't for the lack of a platform SDK and porting existing code, the risk is too great with lack of official support for many commercial SoCs.