I mentioned below how much more American press coverage the Bhutto murder in far-off Pakistan has gotten compared to the assassination of Colosio (who?) in nearby Tijuana in 1994, even though the events were fairly comparable.
One legitimate reason for this could be that we really do meddle more in Pakistan than in Mexico due to Mexico's tradition of anti-Americanism that goes back at least as far as the 1846-48 war.
Personally, I like not being responsible for Mexico.
And it's not as if Mexico would be a better place if we had been allowed to meddle more. The most prominent example of American activism in Mexico in the 20th Century, Ambassador Henry Lane Wilson's conspiring in 1913 to oust the democratically elected president, mild-mannered Francisco Madero, helped set off the bloodiest portion of the Mexican Revolution. This suggests that we would have just made things even worse.