I don't know for sure, but if I had to guess, it's that they are on relatively new, uncommon, or limited architectures for which a variety of compilers just don't exist. Many other compilers depend on being bootstrapped through C and many other runtimes depend on some subset of standard C library functions. Also if you stick to a certain part of C and write very simple code, it can be safe and more importantly very predictable. In embedded usage, you need to know that the execution time can fall within a bound. It can also be important to shy away from getting too high level in architecture or concept because that can hide complex but catastrophic logical bugs.