I'll definitely give credit that a lot of the rest of that post does play a part in the matter, though not necessarily the whole. This, however...often they aren't putting as much effort in keeping things "a secret" as certain folks would insist. The most they usually do is not talk about it on more mainstream media and news, say "no comment" when asked or give fluff, and add "protective laws" that make reporting/showing proof of certain things illegal (like Ag-Gag laws making it illegal to record or distribute videos and images of what goes on in the livestock and dairy industry). The only time they REALLY do any of the erasing that your typical conspiracy theorists claim goes on is when folks catch them tripping up, and then it's often less "people disappearing" and really just a few quick CDMAs or the like to quietly and forcibly take down media about it, or just a few quick clicks if it's on a mainstream media site. Which brings me to the bigger factor....
People just don't want to pay attention. They're happy with their feelgood lives, so they don't want to know about the shit that goes on. When someone gives them fluff about how something's "just a conspiracy theory" they take it readily because it reassures them, reinforces their desired opinion that everything's fine. The mainstream media caters to that as well, only showing or talking about crap if it reinforces or can be presented to reinforce the "desired viewpoint". You want to know what's really going on, unfortunately nowadays you have to go searching for it yourself on the net.
I'll be honest here, the whole thing about conspiracy theories like what Exeter said he dealt with folks handing him, really tick me off; it's the actual crazies that provide a convenient rug to sweep the real mess under. Just call something you don't like to hear a conspiracy theory, and boom, nobody wants it. We all know the Holocaust happened because it was undeniable with all the evidence and survivors, but if the Russians, US, British, etc at the time didn't want it to be found out, they could have just carpet bombed everything and slapped the label on anyone who was actually there. There'd be evidence that remains because they wouldn't get it all, after all the Nazis TRIED to clean up but couldn't get it all before it was found out. But with the same concept of human nature, not wanting to know or believe something that makes us uncomfortable, they wouldn't have to.