It could be because Mexico’s unemployment rate is hovering around 5%, whereas the US’s unemployment rate is hovering just below 8%.
Why the Immigration Issue May Just Fade Away: The Arizona of the future wont suffer from too many immigrantsbut from too few
"...A little-known, but enormously significant, demographic development has been unfolding south of our border. The fertility rate in Mexicowhose emigrants account for a majority of the United States undocumented populationhas undergone one of the steepest declines in history, from about 6.7 children per woman in 1970 to about 2.1 today, according to World Bank figures. That makes it roughly equal to the U.S. rate and puts it at what demographers call replacement level, the point at which women are having just enough babies to sustain the current population. In coming years its expected to dip even further. Other countries in Latin America have experienced a similar drop, though not as sharp. All of which means that the ranks of those invading hordes are thinningrapidly.
...When women start having fewer kids, that means fewer individuals will be entering the labor force two decades later. It has taken longer for that effect to appear in Mexico, however, because even though the fertility rate began falling in the late 1970s, the number of women of childbearing age kept growing. As a result, the pool of newly minted Mexican workers has continued to swell through today. But thats about to change. As soon as next year, demographers say, the number of new entrants into the Mexican labor force is expected to start decreasing. This year that figure is about 750,000, says Félix Vélez, secretary-general of Mexicos National Population Council. By 2020 its expected to drop to 600,000, and by 2030 to 300,000."