Answer:
Mainly due to the xenophobia supported by the racial theories.
Step-by-step explanation:
In the early 20th century, the US received many immigrants, many of them from Europe and Asia. At that moment, racial theories such as Social Darwinism and Eugenics were emerging. In a nutshell, they tried to explain the social problems based on the racial mixture. For the social reformers who based their changes in the racial theories, immigrants were a huge problem. Many of these social reformers were members of the government, and they could create immigration laws. Between 1890 and 1930, the US anti-immigration policy increased its strength and range, avoiding several Europeans, Asians, and Africans to get into America.