Conspiracy theories are popular for the same reasons religions are.
People want to make sense of a world that is, essentially, nothing but chaos. They want to feel secure and in order to do this they want to cling on to this notion that there is something in charge of everything and inevitably this will be sinister.
There's also an additional feeling for these involved and it's based on ego. They want to feel powerful, like they're the ones in the know and only they are smart enough to have figured it all out.