Immigration had peaked in the 1900s. It would probably have continued going up in the '10s but for WWI happening. The quotas in the 20s were meant to limit the number of Southern Europeans, Eastern Europeans, and Jews trying to immigrate in the wake of WWI when the US economy was hurting. Immigration from the Asia-Pacific Triangle was straight-up banned.

There was, of course, a racial angle to it. In 1924 when a more permanent Immigration Act was written, the KKK was about to peak in its membership, and you had people in the Senate saying the US racial identity needed too be preserved and that the Europeans and Jews were arriving to sick and starving to ever be able to contribute to society. I hear the same things now about Mexicans.

It's wild to think that in the 1900s (the decade) when the US population was only 100 million, 10 million people immigrated to the US. We've got similar rates of immigration now, but three times the resident population.

^{edit: I some words.}