I can't be sure, but the fact that libstdc++ is ABI incompatible with the current stdlib probably doesn't help adoption and thus usage and testing. If you want to use libstdc++ every dependency must also be compiled against it, including any system packages.
libstdc++ is gcc's C++ stdlib; libc++ is clang's (although clang can used libstdc++). libc++ supports all of C++11, although there's still some bugs. libc++ is ABI incompatible with libstdc++, but it's explicitly designed to support using both libstdc++ and libc++ within the same program, so that's often not a program.
Sorry, rereading my post I realise I omitted some text (I was on my phone), and shouldn't have said "ABI incompatible". What I meant to say was that the C++11 versions of both libstdc++ [1] and libc++ [2] are effectively incompatible with the C++98 version of libstdc++. The main point still stands: you effectively can not link against libraries using different versions of the C++ standard unless you're very lucky.
