- They haven't used it much in a very long time and have very bad memories of tracing through the completely opaque SGI STL implementation and its derivatives.
- They include std::string and/or std::iostreams as part of the STL, neither of which are part of the original STL and have all sorts of weird warts and poor interactions with the generic algorithm-focused core parts.
Not to say there aren't other reasons people dislike it, but these are big ones.