USC's problem is they're cheap... Urby was making $7mill a year at Oh St, if he were to come back he would command a Saban/Dabo type salary upwards of $9mill a year & more, USC ain't spending that kinda money.
Plus, Notre Dame was always his dream job, he's wanted to coach there since when he was the WR's Coach back in the late 90's under Bob Davie.
ND is one of the few schools that have an unlimited War Chest & will backup the Brinks truck for Urban, USC has money too, but they don't like spending big on coaches, which is why they're program has declined over the years.