Misinforming cabals are one thing, but why antivax specifically and not flatearth musings?
But... it's both? Both of those things exist. And there's overlap, people who believe in one conspiracy theory often believe in others. Someone might just happen to be exposed to one theory as opposed to the other, or they might find one plausible and the other absurd, or it could be that one more effectively appeals to something in one person than another.
My father worked in traffic safety in East Kentucky and something he encountered a lot was people refusing to wear seat belts because they saw it as "authoritarian" for the government to impose it on them. Plenty of antivaxxers frame it the same way, as it being an issue of personal choice and autonomy, against government or pharmaceutical companies. This combines with a natural desire to make sure they're making the right choice for their children, and when they're told that the science is clear and settled and that there's one choice that's objectively correct and another choice that's objectively incorrect, some people don't want to accept that, and they want to get both sides of the story, and as the only way to support antivax arguments is through lies, misinformation, substacks from people called "Dr. Wojak, M.D." they get exposed to that conspiracy theory rather than a different one.
I generally find it questionable to seek primary causes outside of objects that have immanent properties that could explain so much more.
If it was an immanent property of the object, again, you wouldn't see the kinds of variation across countries and over time. We can see a clear correlation that countries that more recently gained access to vaccines have much higher opinions of them. We can find other correlations and come up with reasonable explanations based on that, that for example complacency is a factor. If it wasn't, if it's just an immanent property of the object, then how do you explain the difference between perspectives on vaccines between different areas, like Somalia and France?