Why does the Standard Library for C++ not contain hash table implementations?

I was reading over my text book Data Structures and Algorithms: By Mark Allen Weiss and it says that the standard library does not contain hash table implementations of a set or map, but rather compilers can provide hash_set and hash_map with same member functions of the set and map class. Why not just include hash implementations in the standard library? The book was published in 2006, have there been any revisions of C++ since to add these implementations to the standard library?