America turned towards domestic isolation during the 1920's because of the events going on around the world. A perfect example would be the revolution taking place in the Soviet Union. At the end of the 1910's, the Russian Revolution resulted in the establishment of a communist system. America despised the thought of communism, as it left little room for economic independence/freedom and resulted in the government having too much control over the lives of everyday citizens.
This is why American society was social conservative during this time. American citizens did not want change or outside influences changing the core values that America was founded on. This is why laws like the Immigration Act of 1924 were passed. This law limited the amount of immigrants that were allowed into the United States.