Article: The U.S. Census Bureau reports a 2010 population of 129,779, with 47,094 households and 25,854 families within the city of New Haven. The population density is 6,859.8 people per square mile (2,648.6/km²). There are 52,941 housing units at an average density of 2,808.5 per square mile (1,084.4/km²). The racial makeup of the city is 42.6% White, 35.4% African American, 0.5% Native American, 4.6% Asian, 0.1% Pacific Islander, 12.9% from other races, and 3.9% from two or more races. Hispanic or Latino residents of any race were 27.4% of the population. Non-Hispanic Whites were 31.8% of the population in 2010, down from 69.6% in 1970. The city's demography is shifting rapidly: New Haven has always been a city of immigrants and currently the Latino population is growing rapidly. Previous influxes among ethnic groups have been African-Americans in the postwar era, and Irish, Italian and (to a lesser degree) Slavic peoples in the prewar period.

Now answer this question: New Haven has a large immigrant population, what seems to be one of the main cause of it throughout the last century?
postwar