Definitely because of the cold war.  Reds under the bed and all that.

The funny thing is most of them actually love the socialism, all the benefits that government give them, they just don't want to admit it because of the aforementioned reason why it has a bad name.

ie.  most Americans don't have a clue.   Most of them like socialism but are afraid to label it as such for fears of being called communist.

My personal opinion is that socialism is bad because it messes up people's incentives and leads to less wealth overall.