My hypothesis as to what is the most reliable predictor about someone's willingness to take a vaccine is how much they trust those who are promoting it.

So in this regard the question we should ask ourselves is:

What could be the reason that "media platforms that spread misinformation" were considered to be more trustworthy than those who promoted the vaccine?

I think censorship played a big role in reducing trustworthiness as it's still historically tainted with a negative image of being used mostly by totalitarian regimes. To people for whom freedom of speech is an important value, the attempt of silencing dissent might have been considered as so appalling, that they lost their trust.