USC averaged 4.1 yards per carry. They used a third running back (Carr) who was totally ineffective and who averaged 2.2 yards per carry. Their top 2 running backs (Stepp and Malapeai) ran the ball 32 times for 149 yards, averaging 4.7 YPC. If you're the USC coach and see those stats, all you do, especially while BYU is dropping 8, is run the ball over and over using Stepp and Malapeai. You churn out long, sustained, kill the clock drives and take control of the game.
Instead, he has his freshman QB throw over and over into max coverage and end up with 3 INTs. The dumb decision by the coach not to take the advantage BYU was giving them and leverage it cost USC the game.