Is it really "paranoia" (a mental disease involving ungrounded fears) if the fear is substantiated?
Are you now, or have you ever been, a member of the Communist party?
I'd say the number of non-threats who were actively and vigorously blackballed might call into question as to where the boundary between legitimate fear and paranoia fear is on this topic.
But, somehow, that clear and present danger of Communism no longer played the role it played during Korea War. Why?
Probably no one single answer. I don't think the early years of Viet Nam faced that much ideological opposition. I do think that the political-based mismanagement of the war led to "conventional" opposition to it. Then add in civil rights discontent, the exemptions that made it a "poor man's war" and the general social upheaval of the 1960s, shake well and pour over ice.