The Mexicans know full well that the moment any controls are placed on immigration that either they pack their bags and get over to Europe quickly or they make some very quick reforms. There is no other reason that all prosperity and hope for opportunity comes to a screeching halt when you reach that southern border except that Mexico has an extremely corrupt government and system.
We cannot continue solving all Mexico's vast and growing social problems --- just because they wish to keep things just the way they are.
So, in other words, it is better for a country right next to ours to explode than it is to try to keep the situation under control.
Do you have ANY idea what group would take over once that explosion happens? And what should be done until that mess settles down? Because a LOT more folks will be headed north once that happens. Are we going to turn them back? Will sending people back into a civil war fly with the American people? I doubt it.