Intelligence is much more a function of environment than genetics. One's genetic contribution, in theory, sets an upper limit on intelligence potential, but to actualize that potential one needs a solid education. Someone with genius level IQ born into poverty and with no access to education will wind up just as superstitious and backwards as all of their peers.
This goes doubly so for critical thinking, because critical thinking is not natural. Our brains naturally operate on a survival program that relies on logically fallacies and snap judgments because these are right enough most of the time to keep a person alive out in the wild. This is normally combined with learning from pack mates, which is how our species endured for millions of years. So, without the right education, critical thinking is very difficult for us.
The same is true of scientific knowledge. To someone who has not received the necessary in-depth education to understand clearly why science is better than religion, it looks like just another religion, and even worse, a religion with government backing. Governments are notorious liars, so when you combine that with the unintuitive conclusions of science and no foundational understanding, something like a vaccine looks like an outright trap.
Anti-vaxxers are still wrong, and the misinformation they spread is still harmful, but I must have a drop of compassion for them, given that they clearly haven't had the advantages that I've had.