A lot of the commentary I've seen in U.S. newspapers has expressed surprise that having ditched PRI dictatorship in December 2000 for two consecutive terms under the formerly opposition PAN, Mexican voters would so overwhelmingly switch back to a PRI candidate. If you look at Mexico's GDP under its two PAN presidents, I think this gets a lot easier to understand.
President-elect Enrique Peña Nieto expressed this pretty clearly in a recent Slate interview:
A lesson from U.S. politics is this expression of [Bill] Clinton: "It's the economy, stupid." We need to have a change in the condition of our people. We have not opened enough. ... That is, for me, the key—to grow, to open up job opportunities to the people.
The good news for him is that the Mexican economy has a fair amount of momentum behind it, with rising wages in China restoring Mexico's competitiveness as an export platform to the United States.