The narratives are designed to be earwormy, and everyone wants to live in a spy novel filled with scary conspiracies, so when they hear it, it slips in the backdoor cognitive holes, and they do only minimal "verification" if any at all, and accept it, and then repeat it. It's also very easy to blame an all-powerful adversary for all your woes—did you lose money trading?
Oh, that was gmax's fault.
Did you get scammed?
Oh, that was blockstream's fault. If they'd just did X, then the scam wouldn't have happened in the first place.
Did your friend tell you something secretive that "only he knows" and it's "not public knowledge"?
Well obviously the core devs are behind it.
The absurdity is great when the altcoiners are staring at Bitcoin's behemoth economy with jealous eyes amplifying the narratives they sometimes even know are totally false.
More insidious of course are the lies that are false but simple, where the factually verifiable truth is directly contrary but complex, like the foolish lie about gmax stealing credit, or Vitalik asserting that he was planning Ethereum until the core devs blocked him.
It's easy for outsiders to be infected via their usual lines and information feeds. And, of course, Bitcoin has no marketing department, so obviously defence thereof is a long, painstaking process of careful dismantling and debunking over a long period of time.
Thankfully, the actually helpful, intelligent, and ultra-competent people that are the safest to align with are people on whom these kinds of lies are basically completely ineffectual, so we tend to win, in that respect, just by default.