I was born a long time ago. I don?t recall a usage of the term conspiracy theory before the JFK assassination. At the time of the JFK assassination, the official version of the event was not accepted by the majority of the American public. When the Warren Commission?s preliminary report came out, it was meet by skepticism by the public with the majority refusing to accept it fully.
During the propaganda barrage generated to overcome this skepticism, the term conspiracy theorist came into being. This was a convenient tool. No longer was it necessary to address specific objections to official dogma, now anyone who refused to accept the official version was labeled a conspiracy theorist. This saved a great deal of effort on the part of the propagandists. It was such a convenient tool that it was retained. So from then on, if some one suggested that there might be collusion between governmental, corporate, or both of the above, agencies, the suggestion was rejected as a conspiracy theory.
To this day, this handy tool is constantly in use. For example, if someone notes the cooperation between government and corporate interests in supporting illegal immigration, the immediate response from the propagandists or the propagandized is "Conspiracy Theory".