i had a 412ms cab with sheffields that i played for years, and never thought much of it. i had an opportunity to pick up a 1960a w/g12t75's for cheap. the difference was night and day. i'm not really great at describing tones, but it literally sounded like a thick rug had been removed from in front of the speakers.
so to answer OP's question: probably because they're dirt ass cheap