You need a large, modern C++ compiler and standard library, which are not available for most older systems, and you're inviting an excess of dependencies becaus

You need a large, modern C++ compiler and standard library, which are not available for most older systems, and you're inviting an excess of dependencies because not all compilers support all parts of the newer C++ standards (in the same way), and require a lot more resources and newer versions of APIs and libraries, which further limits their usability on older systems. Furthermore, C89 and C++98 are much easier to bootstrap than a colossus like LLVM and Clang. The few "nice features" are perhaps enticing, but the costs they incur are disproportionate.