As has been mentioned, LA sports fans love winners. Losers? Not so much.
USC has a legacy of winning national championships since the 70's that continued into the early to mid-2000's under Pete Caroll. And although they haven't been in any real contention for a national championship recently, they have had a few seasons where they've held pre-season top-5 rankings. Mix in the fact that they have crosstown (UCLA) and intersectional (Notre Dame) rival games on a yearly basis (not to mention other regular top-10 opponents like Stanford and Oregon), it's easy to see how they can maintain a fan base that will regularly average 65 - 70K in attendence.
Also, you have to take into account that it's much cheaper to attend a USC game than an NFL contest. And if you're not winning consistently, it's hard to justify the $600 - $700 price tag to take a family of four to an NFL game.
