Apparently it used to be easier to get seasonal work permits for Mexicans wishing to work in the US, and it was common for workers to go back to Mexico after the work season. Most did not want to permanently move to the US, but preferred to return to stay with their families. Those visas were curtailed under Reagan, so they became much harder to obtain. Crossing back to Mexico became harder, so now more just cross the border and then never return. If we still had a reasonable system in place to allow temporary workers in, I’m sure we’d see less illegal crossings.