A big reason in the US is that drug manufacturers cannot be held liable in civil court for any death or injury caused by a vaccine produced since 1988. Why do they even need these protections if their product is safe?

If a normal drug is linked to heart attacks or even heartburn the manufacturer can be sued. If a car manufacturer makes a car and finds out later the breaks are bad, they can be sued for crashes. If nearly any product you use doesn't disclose dangers up front and ends up hurting you, you can sue. Yet you cannot sue a vaccine manufacturer for a vaccine related injury.

The fact that many of these pharmaceutical companies have long standing history of greed, negligence, and criminality with liability carve outs they shouldn't really have and you have a perfect storm for vaccine skepticism.

You can blame propaganda, disinformation, lack of belief in science, or social media but even without all that there is still a lot to be critical of.