Since almost all of the operating systems you listed were written in different languages, some specifically designed for a single hardware, the believe of your peers at the time, that C was the only way to write portable code at that time was pretty much correct. This was one of the design features of C. C is also a small, relatively easy to write a compiler for language. That is also a reason why so many mcu manufacturers included a C compiler with their products instead of a C++ compiler, even though C++ is also an old language.