Can you imagine the magnitude of the political shit storm on both sides of the border the first time a US drone operating in Mexican airspace with the blessing of the Mexican government blows up a few Mexican kids?
And unlike Afghanistan or Iraq, Mexico actually matters to the US. They are our #3 trading partner (Canada and China are #1 and #2), even ignoring trade in labor (and trade in drugs).
It is not about firepower. The Mexican army and marines have more than enough resources to kill the Cartel leaders. Almost, every time they have met the Mexican military wins. The problem is finding the leaders. It took 10 years for the U.S to kill Osama bin Laden, because it was hard to find him. The cartel leaders do the same, they are always in the run, it is believed that El Chapo lives in the mountains.
So far, they have captured or killed 24 out of the 37 most wanted drug lords.
I guess our lives and peace are not worth the intervention.