Because of the evidence.
In an unusually objective commentary on conspiracy theories published in the Washington Post, University of Miami political scientists Joseph Uscinski and Joseph Parent, note that two other popular conspiracy theories — The Truther and Birther theories — resonate with only 25 percent of the population. (Respectively, these are the theories that the Bush administration either directed or permitted the 9/11 attacks, and that President Obama was foreign-born and faked his Hawaiian birth certificate.)
By contrast, JFK theories resonate with around 70 percent of the population, according to a recent poll.
“Unlike the Birther and Truther theories, which languish for lack of impartial support, JFK conspiracy theories have some impartial support,” the authors write.
“High-quality information does change minds,” they go on. “When the Watergate story first broke, many dismissed the charges as partisan. Because high-quality evidence became available — hearings were held, evidence was presented, co-conspirators admitted to their crimes — virtually everyone now believes that Nixon conspired to commit and cover up crimes. But unlike the Birther and Truther theories, which languish for lack of impartial support, JFK conspiracy theories have some impartial support.”
They go on:
“Calling it ‘proof’ might be generous, but Kennedy assassination conspiracy theories clear higher evidentiary bars. There is evidence that the government hurried the investigation of the president’s murder and was not eager to find high-profile scapegoats. Most accounts attribute that rush to a pragmatic desire not to wrongfully implicate the Soviets and dangerously destabilize superpower relations.”
“Most notably an official congressional inquiry seemed convinced of a conspiracy, so even those with moderate conspiratorial predispositions have something to hang their hat on. Because of their evolving scapegoats and above-average amount of evidence, JFK conspiracy theories have lodged themselves into the collective consciousness.”
I think Uscinski and Parent have it right. JFK conspiracy theories are popular because there is corroborating evidence, much more evidence than supports improbable theories that hold 9/11 was an “inside job” or Obama was born in Kenya.