It's a good question. As Dave Anderson observed at Ian Welsh recently:
One of the options for managing violence in northern Mexico is for the government to embrace a most favored cartel (mfc). Since at least April of this year, the Sinaloa cartel has been rumored to be a contender for the spot of the most favored cartel. The argument is that there is a tacit agreement that the MFC and the Mexican government would cooperate with each other to suppress other cartels. The MFC would agree to divert some of its kickbacks to the relevant governmental elites as well as maintain urban security with a tolerable and much lower level of violence as its competitors would no longer be alive or competing with it.
Most Favored Cartels And Car Bombs
The Mexican government, in short, cannot stop all the cartels from operating, so they have picked one to back, and let it either absorb or kill off the rest. That's what all the money we've sent to Mexico for marijuana has helped create: a state that can't even fight its own criminal elements.
This violence threatens to spill over into our country, but our politicians mostly don't have the nerve to say this is true.
Our current drug policy, particularly regarding marijuana, is madness. All it does is make the domestic prison industry happy, and corrupt the governments of our neighbors and allies.