After the assassination of JFK, there were many people who didn't buy the original narrative that Oswald acted alone. The CIA chose to label these people as conspiracy theorists in order to discredit them.

The CIA has a huge influence on the mainstream media. In the 1950's they began Operation Mockingbird to turn MSM towards propaganda. One way they did this was through the blackmail of journalists.

Today, 90% of MSM is controlled by only 6 companies. http://www.businessinsider.com/these-6-corporations-control-90-of-the-media-in-america-2012-6 This creates an illusion of choice for the individual that doesn't truly exist.

These corporations control what is seen in the media today. A conspiracy theory already starts with a negative connotation but can also be spun as "debunked" by the MSM in order to further discredit it. Once one MS source runs an article, several other sources will pick up the same story and run it since they're owned by the same companies.

This narrative will appear to be the truth since so many sources will be running it. The average reader will trust the media since they're unaware of its nature.