Post the Great Depression, the USA has had a pretty good run as far as a high standard of living for its residents. That's true even with the numerous recessions that have occurred over the decades. Everybody in the middle class has two cars, color TV's and big houses. America is the land of plenty.
I think all this economic prosperity has given Americans the luxury to sit around and dream up things to be afraid of. The populace isn't concerned with thoughts of their next meal or hyperinflation. Instead they deal with their boredom by manufacturing fears. Some of these topics are legitimately worth being concerned about (though likely not to the degree we've obsessed over them as a nation). Others were simply paranoia:
1950's = Communists
1960's = hippies
1970's = industrial toxins in environment give us cancer
1980's = Communists again under Regan
1990's = crack epidemic / drug wars
2000's = terrorists