Washington Post, February 26, 2014:
The cartels move drugs from Mexico to the US, where they have extensive networks to distribute/sell the drugs. Those networks necessarily include enforcers who are willing to threaten, intimidate, injure, or kill anyone their bosses decide needs to be threatened, intimidated, injured, or killed.
The reason those networks and those enforcers don't wage any kind of general war on the US government or the US populace is that it doesn't make good business sense.
They're doing just fine at getting the drugs in and selling them profitably without gunning down cops, bombing government buildings, etc. Law enforcement activity, to the cartels, is just a cost of doing business, like the inevitable shoplifting losses in normal brick and mortar businesses. If they picked an all-out fight instead of relying on avoidance and bribery, it would get a lot more expensive, a lot more dangerous, and a lot less profitable.
But what happens if the US unleashes its drones and special operations forces on Mexican wedding parties, or even sends in a conventional ground force to play Tora Bora "root them out" games in this or that area?
Well, maybe some of those enforcers get orders to buy some timers, pack some vans full of ammonium nitrate and nails, and leave those vans parked across the street from police stations.
Or, for that matter, middle schools.
This is not a fight any sane or moral human being would want to pick in the first place, let alone continue after the first mass killing or two.
For one thing, it's not "winnable." Even if these particular organizations were "defeated," new ones would spring up to take their place (Colombian cocaine production has, according the US government, tripled since the Medellin and Kali cartels were "destroyed").
For another, the "terrorism"-related body count inside the US would make the period starting with 9/11 and running through now look like the good old days. We're talking about people who stack pyramids of severed heads in town squares when the Mexican government pisses them off.