U.S. reliance on immigrant labour is key to this discussion, and strongly colours all elements. It is a direct legacy of early slave labour (Blacks in the East, native Americans in the West) as well as indentured servitude. Importation of an underprivileged labour class dates to the first days of the Republic as well, and includes the Germans, Irish, Italians, Greeks, Chinese ("coolies"), Mexicans ("Braceros"), and in the post-WWII era an increasing number of Central and Latin American immigrants.

By 1) relying on such labour and 2) denying either lawful entry or any plausible path to permanent status for the vast majority of these workers and their families, not to mention many, many, many broken promises along the way (asylum, DREAM Act, don't get me started on Native American treaties), the U.S. has de facto maintained a legally-underprivileged and disadvantaged working class long after the statutory prohibition of (most) slavery and indentured servitude.

Yes, there are also economic and political circumstances within the US geopolitical orbit which encourage inward migration. That too cannot be separated from long-standing U.S. foreign, economic, and military policies within the region.