It's a combination of things. The first is the lack of expertise in existing companies and lack of available hires. Rust is new and has a steep learning curve. I work in a company with 7k+ engineers. Our Rust interest group has less than 20 members and most are beginners. I have never seen Rust mentioned on CVs even once when hiring for Embedded positions.
The second reason is lack of tooling - specifically Functional Safety toolchains that can be used for ISO26262 projects. There are plans by Ferrous to develop one, but it will take years to gain any adoptions [1].