What makes you confident that is the reason? Are all people equipped to work out what is obviously false?
It is one thing to be distrustful of official sources and another to believe that Bill Gates is injecting nano particles into vegetables based on a video you saw on YouTube.
People don't just start believing Bill Gates is injecting nano particles though. They get there through a gradual process.
Edit: And I am confident this is the reason because I have observed people go down this process. People who are intelligent, and not the caricatures they are made out to be.
You're right - the Bill Gates injecting nano-particles into COVID vaccine has its basis in several pre-existing conspiracy theories, such as anti-vaxx, fears of RFID/"Mark of the Beast" tagging, Satanic/ritualistic activities by elites (see: Bohemian Grove, Skull and Bones, etc.) which themselves are derived from (albeit not always directly indicative of) anti-Semitic conspiracies.
Even with QAnon, the pieces were already there among the right-wing and extremist Christianity, they were just assembled into a single, somewhat coherent narrative.
It is one thing to be distrustful of official sources and another to believe that Bill Gates is injecting nano particles into vegetables based on a video you saw on YouTube.