In the past most of the Cartel money is spent on the Mexican side of the border. Mexican banks are washing the money, why would they want to stop that money from coming in?
Now it has gotten out of hand, Killing of thousands of people cannot be simply ignored, but it has become so widespread and so many are either frightened or bought that the Government has become a joke.
I am sure Mexico would love to stop the killings, but they still want the money. A quandry.