Realize that while the United States may not have been the definitive 'victor' after World War II, they still emerged as one of the most powerful countries in the world, with a broad spectrum of influence. The United States saw it as imperative for other countries to rebuild their post-war economies and political structures as democratic and capitalistic in nature, because it affirmed the idea that the American way of life was supreme.
To have a 'kink' in the system (i.e. communist sympathizers) defied the very nature of what made America 'successful' after the war. Countries like China and Korea demonstrated the threat of communism abroad. Politicians seized the opportunity to criticize communism in other countries as well as at home as a threat to American ideals.
Why do you think the McCarthy wave caught on so quickly? Why were initial 'anti-American' investigations targeted towards the Hollywood industry, and then progressed into the government and army? Why was there so little resistance until the army-McCarthy hearings?