Some blame the rise of the Internet for America's obsession with conspiracy theories. Others, usually historians and social scientists, blame this obsession with conspiracy on the increasing lack of transparency within the United States government, starting around the turn of the twentieth-century and climaxing during the Cold War and the global War on Terror.
In many cases, both sides are right, and wrong.
Yes, the rise of inexpensive Internet access created the conditions of possibility for a culture obsessed with conspiracy theories, so did cable television and, of course, other forms of media. The Cold War exacerbated the conspiracy culture ingrained deep within American society and culture. However, grand conspiracies have been part of the larger American psyche for much longer than the twentieth- and twenty-first centuries.
In fact, cries of conspiracy can be found within America's founding documents. Particularly, one can find a rather interesting conspiracy theory charged against King George III (r. 1760-1820), who is said to have conspired with indigenous peoples to undermine colonial interests in the American colonies, especially in the Colonies' move westward.
He [King George III] has excited domestic insurrections amongst us, and has endeavoured to bring on the inhabitants of our frontiers, the merciless Indian Savages, whose known rule of warfare, is an undistinguished destruction of all ages, sexes and conditions. –U.S. Declaration of Independence
The nature of grand conspiracies don't end there.
Another conspiracy, one that has gone down in the history books as the Constitutional Convention, probably helped cement, at least at a later date, the notion that machinations of conspiracy were constantly afoot. The Constitutional Convention, convened in May 1787 and adjourned in September 1787, met to fix the problematic (and, if we're honest, utterly useless) Articles of Confederation. However, as the story goes, the statesmen decided to scrap the Articles and decided to work on, in secret, a new document, the U.S. Constitution.
Jesse Walker, who argues that conspiracy theories aren't to be ignored but studied, articulated in an interesting interview on Reason.TV that conspiracy theories have been a staple of American political discourse and history since before the United States existed as a nation.
The grand conspiracies continue the deeper you delve into American history: Conspiracies involving foreigners, communists, terrorists, former slaves, Mormons, Nazis, mega-corporations, and so on.
What makes this conspiracy culture so prevalent?
It's hard to say. Some conspiracies have been known to be true, such as U.S. government mass surveillance of U.S. citizens, and others, many (many) others, rightfully so, have been discredited. Nevertheless, conspiracy theories are abundant, and it is unlikely that conspiracy theories are going away any time soon. With the barrage of lightning bolts like freedom of the press, inexpensive publishing, and cheap Internet access are likely to be the greatest feeders into the conspiracy culture of the U.S.
The only way to defeat conspiracies would be effective education in facts, inferences, misinformation, fake news, etc. These are things our schools need to be teaching to all ages, including our most vulnerable populations, such as those over sixty-five. In other words, media literacy needs to be at the heart of our curricula, no matter the age, in order to ensure that vulnerable populations don't succumb to conspiracist thinking.