The historical answer is because Congress wanted to be sure that employers could fire Communists for being Communists.

Of course, that's not my view. I think political affiliation should probably be protected, but it needs to be very narrow. You shouldn't be able to be fired for being a Republican. But if you post "Gay people should be executed," you shouldn't be able to hide behind "I'm a Republican, that's a political view!" any more than you should be able to hide behind "I'm a Christian, that's a religious view!"