What would be a better language for creating a small, portable crypto library that works on embedded systems? Would you really put your trust in a brand new language?
Also, when I say that BearSSL is written in C, it is partly a lie: some of it (especially X.509 certificate decoding, and handling of handshake messages) is done in T0, a new Forth-like language that I invented for the task (compiler is provided, and produces C code), specifically to have coroutines (incidentally, it means that most of that code read things byte by byte, with relatively few accesses to buffers, thus less potential buffer overflows).
Despite the horrors implied by a Forth syntax, this actually made writing the code easier. As a bonus, it turns out that T0-generated code is very compact, so I could pack more behaviour into less bytes.