What most encourage Mexican immigration to the United States is the possibility of getting a job. The economic situation in Mexico is very bad and applies mainly to poorer people.
The United States has millions of options for immigrants to work, sometimes they are not well paid and are inhuman, but the chance of getting a job is more important for them.
This affects Mexican economy positively as emigrants (living in the U.S.) are able to help their families that still live in Mexico and diminish the levels of poverty.