SDL_Mixer and SDL_Sound

Why are SDL_Mixer and SDL_Sound maintained separately, with SDL_Mixer being
the standard relatively ‘core’ library to be used with SDL?

I understand that Ryan Gordon contributes a lot to Mixer, as well as being
the coordinator and ‘main-guy’ for SDL_Sound, which seems to have a very
complete api. I was just wondering why SDL_Sound wasn’t officially the
audio library to use. That way SDL_Mixer could be retired. I don’t mean to
offend anyone if I’ve missed important features of Mixer (maybe having less
functionality is important for stripped down access?) or simply
mis-represented the situation, but I’d love to know why both libraries are
still around.