This is a very interesting topic, and a bit of a can of worms! Thanks for producing a very thought-provoking post once again.
The key is, as I think you mention, who decides what is a conspiracy theory and what is fact? It can't simply be "science", as scientists often disagree. It's easy to say that the belief that the world is ruled by a shadowy group of people is a conspiracy theory, because that simply cannot be proved. But what about not believing the "official" explanation of 9/11? People who question the official government line are often described as conspiracy theorists, whether they have fixed opinions on what actually did happen, or whether they are just researching to get closer the facts.
For this reason, independent thinkers like James Corbett of @corbettreport are often tarred with the "conspiracy theorist" brush, yet he sources his work more thoroughly than most mainstream media outlets.
I am a former magazine journalist, taught to question things. I was drawn to the profession because I've always had a questioning mind. My parents used to say in exasperation, "you could argue black's white"! This is because I have a habit of looking at things from many different perspectives, examining things. And it often puts me in the "conspiracy theorist" category, because I never, ever swallow the government line. I don't take any line. I question everything.

People who feel socially marginalized or unhappy with their lives are more likely to believe in conspiracy theories than those who are satisfied, happy, and fulfilled.

I think there's a reason for this. It's when people actually are socially marginalised – for example after the recession of 2008, when many people lost their jobs – that they start to question the things they used to take for granted. And if those people are not critical thinkers, it can lead them into all sorts of weird beliefs.
But if those people are critical thinkers, it can take them into places mentally that other people fear to tread, and then they are often tarred with the "conspiracy theorist" brush, because they are daring to challenge authority. One example off the top of my head is John Wedger, the former detective constable who has spoken up about under-cover child sexual abuse.
There seems to be a concerted effort going on by the "authorities" to characterise independent thinkers as "conspiracy theorists". I'm referring to things like fake news, the "prop or not" fiasco (which described the work of Dr Michael Greger as Russian propaganda!), Newsguard etc. I think this is partly due to the internet giving many people a means of getting a public audience and thereby competing with mainstream media, and partly due to public anger and questioning of the wars in the Middle East, and bankster behaviour during the recession.
Maybe this is a conspiracy theory, but I think governments are desperately trying to put that genie back into the bottle.