Conspiracy theories depend on the size of the concept behind them.
They can be planet-wide - climate change for example, or individual, the death of Elvis or Diana, but they are never small because they must involve a concept that is ultimately beyond convincing proof.
If you can raise a theory about something that chimes with human conciousness, and taps into fundamental insecurities about powers over which we have no control - governments, climate, the Royal Family, what ever, then you can expand it by using human nature, which is always to look for certanties and the security they bring.
It is human nature to enjoy rational explanations, and the seed that the rational explanation is wrong, even without firm facts and evidence to back up that contradicition can blossom into a conspiracy theory.
My personal belief is that religion and conspiracy theories occupy exaxtly the same area of human experience and thought - the need to find an order and a reason for the things that concern us all.
Proof is never going to be found - with that the theory, or the religion would vanish, but the definition of faith is the ability to hold a belief that cannot be proven. That balance between firm belief and absence of evidence is what makes religion work - and conspiracy theories can ride on that identicl psychology.
Somehow, the essence of faith and belief suts the human psyche - without that feeling of comofrt and support that minds gain from those factors, neither religion of conspiracy theories would exist - but they do, and have done so since man's ability to think cohernetly about aspects outside simple survival.
Since our mental sophistication is only ever going to grow, you can rest assured that these twin comfort zones will continue to grow as well.