It’s not actually in the US’ national interest to have Mexico be a functioning state. How would we be able to get cheap seasonal immigrant labor, or export manufacturing jobs if they were? Perhaps more importantly, if Mexico really was a functioning economy we would then have to compete with them, rather than profiteer off of them. There is very little geopolitical incentive for the US to fix any of Mexico’s endemic problems, in fact it’s probably a far more in the US’ interestes to maintain the status quo making sure that no one faction ever gets too powerful.