Called the Emergency Quota Act, the establishment of "quotas" for immigration emerges out of eugenicist panic, and focuses primarily on reducing the number of Catholic Europeans, which is why immigrants from northern/Protestant areas are favored, and other immigrants from the Western Hemisphere.
This focus on Catholic Europeans over Protestant Europeans is an expansion of white eugenicist pressure in the United States to "protect" the "Anglo-Saxon race." The immigration bans of 1907 and 1917, which were the first comprehensive immigration laws in the United States that focused on Asian countries, also remain in effect during this period.