Maybe they don't go further back. The State Dept. certainly dragged its heels in implementing the warnings; they should have been in place in 2006 when the violence began to ramp up following the beginning of Felipe Calderon's crackdown on the drug cartels. Here's what I found:
Though many must go there on business or for diplomatic concerns, these reports should be enough to deter vacationers until the violence is largely quelled. I think political correctness likely delayed the implementation of these travel warnings.