Let's see. I don't think it's farm subsidies. In some parts of Mexico, emigration has become a rite of passage. I forget the name of the town in Mexico, but there it is almost unquestioned that once a person reaches a certain age, they will emigrate to New York City. The original reasons are probably the same as any other immigrant, but over time it has become viewed as their right to illegally immigrate.
You also hear anecdotal stories of women whose children have chronic illness or diseases like leukemia who will emigrate to the US for better healthcare. There's also pollution caused by maquiladoras, and the rising birth defects that Mexican women KNOW are caused by their work in the maquiladoras. They can get paid a pittance to work with and near toxic emissions, or they can cross the border and make cheap T-shirts for twice as much money. It's still hard labor with long hours, but given the choice...
A photo of the Rio Bravo post-NAFTA. Would you want to live with this in your backyard? btw, the river is called the Rio Grande in the US and the Rio Bravo in Mexico.

I will revisit this thread later, I've been up all night with homework. There's an excellent book I can't think of the name of that follows one Mexican illegal immigrant in his crisscrosses back and forth over the border, a microcosm of the larger picture.