Professors Barbara Walter and Alberto Díaz-Cayeros over at the Monkey Cage:

Their strategy of intimidation targets mayors, police, prosecutors, journalists, and pro-regime citizens. The cartels’ objective is to demonstrate that the government is too weak to punish them or protect future victims.  In fact, violence helps elicit the tacit support of the population, which is why Mexican drug organizations now operate openly rather than in secrecy.

This intimidation strategy also thrives on uncertainty: can the government protect me? With Mexico’s two deadliest cartels employing more than 100,000 foot soldiers combined, it’s not always possible to know who is in charge in a given area.

That sums up the problem pretty well. For their answer, see here.