It is because Pearl plays so many players. Downy and Raley-Ross played 39 and 38 minutes each. They scored 15 and 16 points each. But their points came in the first 30 to 33 minutes of the game. By the 7 minute mark of the 2nd half, they were gassed, as was all of USC's "go to" players.
Chism and Prince played 28 and 21 minutes each. They scored 11 and 10 points each and when the game was coming down the stretch, they were fresh and could make play after play defensively, on the glass, and on the offensive end.
USC's players were tired and tired players take easy shots -- usually from three point range -- but their legs are gone and the result is a lot of misses.
That is why UT plays flat but often pulls out wins down the stretch against teams that on paper you would think we would beat by 20. The UGA game is an even better example. They looked very good in the 1st half and then ran out of steam in the 2nd.
The problem with this approach is that it doesn't work against good teams unless you have a lot of talent on that bench. If we are playing Pearl and McBee a lot in March, we are going to lose. We'll do fine in the first round or two of the SEC tourney, but in the NCAA tourney, playing 2 games in 3 days is not that big of a physical challenge to teams to make that big of a difference.