Why does it tend to take american citizens several decades to romanticize the immigrant?

I hope Chivington and Fetterman Massacres

Americans have always tended to romanticize the immigrants of their grandparents' generation while casting a skeptical eye on contemporary newcomers. In the first decades of the 20th century, descendants of Northern European immigrants resisted the arrival of Southern and Eastern Europeans, and today the descendants of those once unwanted Italians, Greeks, and Poles are deeply distrustful of current immigrants from Latin America