Following world war l, why did congress limit immigration from countries in southern and eastern europe?

WWI had many disastrous consiquences, in europe many of the citys and residential areas where destroyed or leveled by bombs, but the US was not leveled like many of the european citys were so the US most likely limited this to keep the US from being crowded with "refugees"  and the like.

Congress was responding to the activists' calls to "Keep America for Americans