Because a gameplan like 2012 would include a semi-competent defense, which we lacked for the second half of the year. Running the ball more against USC wouldn't have meant a win. It would have meant we could have possibly narrowed the margin of defeat (big maybe). They still would have scored on most of their possessions...they just may have had fewer posessions.
Further, if you wanted a gameplan like today, it necessarily includes a quarterback who is a threat to run the ball in the zone read. The change in gameplan was slightly more complex than "let's run the ball a bunch more".