Education, yes all parents want that for their children and access to exceptional medical care--so for those reasons combined they seek our America. Perhaps this is where privately run [aka corporate] education would benefit over government owned. "Opening soon a new school by you." Worldwide projection.
And perhaps I am painting too broad a brush stroke as well..but I do think being in poverty with a neighbor who seems to have all the riches makes one want to reach for the same golden ring.
Past Mexico's President Fox granting duel citizenship was the cherry on top.
But you follow the money and the money is in America.
When all the world puts down the US, it is noted by conservatives that we are the one country in the world where all the world wants to live, and no one wants to leave.