The 9/11 terrorist attacks were some kind of curious turning point about conspiracy theories. Or more precisely, about the attitude people have towards them.
You see, before that the attitude the general public had towards conspiracy theories was more on the side of amusement. "Those crazy tinfoil-hatted conspiracy theorists..." However, after 9/11 believing in all kinds of conspiracy theories has become trendy. Not only has their general acceptance among the general population increased significantly, the theories have also gained a lot of visibility due to celebrities (especially in the United States) advocating them on TV. Nowadays it's fashionable to believe and promote conspiracy theories.
Of course conspiracy theories are nothing new. They have probably existed for as long as humans have. However, for some reason only the "modern" conspiracy theories are popular and trendy, not really the "classical" ones. These "classical" conspiracy theories are related to things like the New World Order, the Bavarian Illuminati, freemasonry, The Protocols of the Elders of Zion, the Bohemian Club, etc.
While these are not as popular, they are still promoted by many people (such as for example radio host Alex Jones).
One of the major problems with these old "conspiracies" is that they don't make much sense. Why? Because they are all related to world domination, yet they surely are taking their sweet time to do it.
Consider that the Illuminati was founded in 1776, the Freemasons were probably founded somewhere in the 16th century or even earlier (maybe as early as the 14th century), the Bohemian Club was founded in 1872, and The Protocols of the Elders of Zion was published in 1903 (and is based on a French book from 1864).
There have been real conspiracies in the history of humanity, of course, but one common feature to them is that they are relatively short-term. For example there have been state leader assassination and coup d'etat conspiracies during the entire history of humanity, but their duration has invariably been from days to at most a few months. Likewise there has been secretive actions taken by governments, for example to spy on their own citizens, but likewise these actions have always been almost immediate, with a planning stage of at most a few months.
There are many reasons why global conspiracies spanning hundreds of years don't make much sense. For example:
World domination plans require strong convictions, people to be very passionated about it. Such strong convictions and passions are in stark contrast with the plan spanning centuries, advancing very, very slowly. What's the point?
This can be seen throughout history. For example Alexander the Great wanted world domination. He didn't wait for centuries. He became the creator and ruler of one of the largest empires in ancient history by the age of thirty. Adolf Hitler had world domination plans. He didn't plan for it to happen slowly in 300 years. He planned for it to happen in a few years of active conquest.
Conspiracy theorists will argue that those rulers were defeated because they were too hasty, and that the idea behind the great conspiracies is to conquer the world more nefariously, without people even noticing. But who exactly benefits from this? Who has the motivation to promote a conspiracy that lasts for centuries?
There's basically only two things to gain from conquering the world: Money and power. If someone wants money and power, he wants them now. What good does it do to him that perhaps 300 years from now some strangers who haven't even been born yet (and who will most probably not even be related to him) might get the money and power he worked so hard to gain?
And of course there are many other problems with the whole idea, such as the ones I describe here.