Give me your tired, your poor,
Your huddled masses yearning to breathe free,
The wretched refuse of your teeming shore.
Send these, the homeless, tempest-tost to me,
I lift my lamp beside the golden door!
The United States is our land. If it was not the land of our fathers, at least it may be, and it should be, the land of our children. We intend to maintain it so. The day of unalloyed welcome to all people, the day of indiscriminate acceptance of all races, has definitely ended.
So what changed in less than two score years? You could play the Turner thesis card and say the frontier had closed, but we still have anti-immigration laws and sentiment even though places like North Dakota have plenty of room left (35 of the 53 counties in North Dakota qualify as frontier (less than 6 people per sq mile according to Turner)).
Labor unionization efforts started succeeding in the 1880's (Gompers founded the AFL in 1886). Increasing labor political power would serve as a significant motivator for politicians, and at least the AFL actively strove to get laws like the Emergency Quota Act passed. Additional union support may have come from the ratification of the 17th Amendment in 1913.
Some unions may even have net negative effects on society. If we include the lost economic productivity due to unionized wages and lower immigration rates, even more unions would swing into net negative territory. Given the legal protections for workers in the US, perhaps we should re-examine the legal protections enjoyed by unions?
Fun trivia question: What country's population consists 100% of immigrants? Don't think big ;)