The short answer is that for most of our (United States and Mexico) joint history, Mexico has not systematically attempted to patrol its side of the border. The country has no border patrol and the military is deployed only in crisis periods and at specific points. Security issues along the border develop more problematically because of past failures by both governments to deal with infrastructure and migration. The illicit drug market itself is not sufficient cause for the large-scale violence wracking Mexico today.
http://www.fpri.org/pubs/201102.danelo.geopoliticsofnorthernmexico.pdf
