Answer :
One reason can be that they didn't trust the good intentions of foreign countries. The reason America was colonized and fought for its independence is to get rid of the foreign countries from Europe who only wanted to exploit them. Another reason was that they feared that someone else might take over their country. Example for that is when Kennedy was becoming the president and many conservatives believed it was a way for Pope to take over the country because Kennedy was Catholic.
Simply because they were different
They did not fit with american government and society
Socialist and anarchists were unpatriotic (opposed the war)
Anarchists believed in violence and had targeted americans with bombs
After the war, americans were distrustful of foreigners and their ideas