But I don't think it's super mysterious. The US is one of the few countries that has a major news network constantly pushing the antivax message (Russia being the other that comes to mind, and that strengthens my point). We also have an erosion of trust in institutions, the media, and authority that goes deeper than in most other places.
We also see evidence here on HN: every Covid thread has a significant number of antivax comments. They are also aggressive on Twitter, and even in the chat of TWiV livestreams.
There is some reason for optimism though. Very recently, there has been something of a turnaround in right-leaning news sources, where they are now advocating the vaccine. I'm not sure if that will hold up, but in any case I expect it will boost vax numbers considerably, and in the process save many thousands of lives.
A deeper question remains: what can we do toward having an information ecosystem that is not so terribly dysfunctional that antivax messages thrive in it?
[1]: https://www.theatlantic.com/health/archive/2021/07/unvaccina...