I'm not a part of the standards committee, but here goes:
strptime is defined by the ISO/IEC 9945 documents (essentially the Single Unix Specification / POSIX). It is not part of the 9899:1990 or 9899:1999 C standards documents (and I don't think it is part of 9899:2011, though I can't be sure as I don't have that one in my possession yet).
Since the C++ standards only mandate levels of compatibility with their C cousin, you won't find strptime mandated by the 14882:1998 or 14882:2011 C++ standards documents.