I feel the physical structure (design) plays key as to how loud stadiums are. BDS starting out as a bowl, (similar to Michigan) makes it more difficult to get the sound forward/down. BDS can get loud, but stadium just isn't designed to make it deafening.
I remember years ago Herbstreit was in Oregon calling a game and mentioned how impressed he was with the crowd noise at 52k. It is a bowl style as well, but I guess the design is such as to face you more forward. IDK.