And then there is a the question of portability. There is a C compiler for virtually anything powerful enough to support a compiler. C++ rules out some of the smaller microcontrollers but is available virtually everywhere else. Sure the code may need some work to remove platform specific assumptions, deal with missing library functionality on very small targets or deal with compiler bugs but that is still likely to be much easier than a complete rewrite.
Any new language that is intended to be compiled to native code (either at compile time or at runtime by a JIT) has a massive uphill struggle to come close to the portability of C/C++. AIUI GDC only got proper support for arm linux very recently and is still missing proper support (e.g. the libphobos standard library) for many architectures.
Scripting languages have things a bit easier because they usually piggyback on the existing C/C++ compilers. So the developers for the most part only have to worry about OS portability, not CPU portability.