I think three reasons can trace USC’s decline in scoring in Pac-10 games from 2002-05 to 2006-08.
1. Norm Chow. 2. Reggie Bush. 3. The decrease in takeaways.

Here’s some stats from the Morning Buzz:
USC’s takeaways decreased the past four seasons. The Trojans averaged 38 per year in Carroll’s first five seasons. But in the next three seasons (2006-08), USC averaged only 26.
USC’s offense scored 30 or more points in 27 of 32 Pac-10 games between 2002-05 but did it only 11 of 31 times from 2006-09.