Why Immigrate to the U.S.

Mexico is not that poor a country, actually far wealthier per capita than a BRIC giant like India. The U.S. in the best of times is not a pleasant place to be working class in, with a scarcity of subsidies for things like education and public transport, plus a brutally low minimum wage.
There is much less reason to immigrate to a country with chronically high unemployment, a weak safety net, and tepid growth. Most of the world is really still booming. It's not just Mexicans who are finding grass greener on their own side of the fence.