Answer: because of the massive subsidies to grain farmers in the US by the federal government. Go to Google, type in "immigration and corn subsidies," and check it out. The following are typical of what you will find:
Remember NAFTA? That's when the big problems began.
Hmmmm! Why don't we cut off these subsidies?
a. failure of the political process
b. corruption of the political process
c. a and b
d. all of the above
Too simple? Yes, and some progress is being made, But we are many years too late.
PS Senator Obama of Illinois voted consistently for corn subsidies. (Iowa and Illinois are the two biggest corn producing states.)
See also post of October 15, 2009.