There is a lengthy article about the origins of the Mexican drug cartels by Carmen Boullosa and Mike Wallace at Jacobin that is well worth a read (though it is not a new argument). They argue that economic reforms pushed by the United States are responsible. The lost decade of the 1980s, which led to U.S.-imposed free market policies, opened up new spaces for drug traffickers to flourish. Here's the crux of it:
Farmers, unable to sustain themselves due to the removal of subsidies and the arrival of competition from US agri–corporations, found the burgeoning market for marijuana and poppies their only avenue to surviving on the land. The army of the urban unemployed gave the cartels a deep pool from which to recruit foot soldiers, and the miserably paid (and eminently corruptible) police and military provided the muscle with which to protect their interests.
The spread of everyday crime — aided by the rapid declension and corruption of local police forces — demoralized civil society, and provided a climate within which grander forms of criminality would flourish.
The adoption of free trade, and the deeper integration of the Mexican economy with that of the United States, dramatically increased cross-border traffic, making it far easier to insert narcotics into the stream of northward–bound commodities. Some NAFTA rules were of particular help: because maquiladoras were exempt from tariffs and subject to only minimal inspections, Mexican smugglers began buying up such factories to use as fronts for shipping cocaine.
Narcotrafficking had formerly been integrated into the PRI corporatist state, an under-the-table equivalent of labor, peasant, and business organizations. As such it was subject to a certain degree of regulatory control, and to unofficial taxation, in return for the de facto licensing of smuggling (the plaza system). The state’s abandonment of this form of corporatist inclusion contributed to the independent growth and power of organized crime syndicates.
The quibble I have is that although the end of the one-party is mentioned, it's not given enough attention. The article is really focused on criticizing U.S. policy, and the PRI's downfall does not match that theme.
This is important because it raises the question of whether the "drug wars" would have evolved in the same manner if the PRI had retained hegemonic control of the state. After all, the PRI did keep a lid on drug-related violence even through shock therapy and the implementation of NAFTA. Once the PRI's grip was released, political power vacuums opened up and drug traffickers quickly moved in. Then the free flow of cross-border exchanges made it easy to expand.
Either way, there are no happy conclusions. Free trade isn't going anywhere, and Mexico is a multi-party state. Both are good for Drug Trafficking Organizations.