Governments and their plans...
Why don't these socialists just free up their economy so that enterprising Mexicans can take advantage of high prices to plant more corn. Duh!
Agriculture is a very specific branch of economy. Farmers need stability and predictability of prices. That is why subsidies/regulations/quotas may be needed.
When farmers abandon farms and go to city, they cannot return. Every nation needs some degree of food production secured.
I think one of the problems with that is that no one knows what corn prices will be like this year. They can only speculate. American farmers will likely plant several million more acres of corn than were planted last year and if the weather is good and they have a bumper crop the world markets could again be flooded by dirt cheap American corn. Mexican corn farmers are not subsidized like American corn farmers. If the bottom drops out on corn prices our government will pay out enough in subsidies to keep our corn growers in business. Mexican corn growers would do fine if prices remain as high as they are at present, but if prices drop back down as they may very well do, Mexican corn growers will lose their shirts. They're afraid to plant too much corn.