90,000+ seat stadiums
USC Coliseum: 93,600
Rose Bowl: 90,888
and, as others have pointed out, there is not a strong “college football culture” in Southern Cal as some other places, due to a greater variety of other attractions/things to do, such as pro sports, tripping to Vegas, world class amusement parks, going to beach, etc. Also, I think there are a lot of transplants there from all over the US and they are not as loyal to the SoCal teams due to loyalty to their home state teams — with all the sports networks and streaming one stay home to see their hometown team(s) play rather than attend live football at SC or UCLA.