Well, a simple language has some capabilities that are important in the embedded space that a complex language lacks, like the ability to quickly write compilers and to do both formal and informal analyses. It can also be learned more quickly, which is very important for adoption. I don't think that any of those is an immediate deal-breaker (Ada has had some small success in the embedded space despite being complex), but some preferences tend to be more common in some industry niches.

BTW, I'm not disappointed with Rust for some "slight discomfort," but because it doesn't even begin to address my main discomfort with C++. Rust isn't my cup-of-tea for the same reason C++ isn't (and I program C++ all day, every day).