It's really not in American interests to have a wave of violence destabilizing a neighbor that shares a border. China wants the Panama Canal, or something like it. China also has lots of mind bending chemicals it can dump into America, to slow things down economically, while profiting from the trade.
America doesn't want violence in Mexico because it fucks things up for Americans. China, on the other hand, does not give one single fuck about what happens in Mexico, because it's far away, and there are no direct effects.
When an external actor is capable of fanning these flames, it also shows Mexico how powerless America is, as far as keeping allies safe. America isn't the trouble maker here. The violence is pointed at America, given America's involvement in other places, like the middle east, as a check to those American power plays.
Is this not quite obvious? Just look at the geography, and the rest follows.