In The Crucible, why does Danforth hesitate to accept Mary Warren's statement that her prior testimony was pretense?

Simple. In that day and age, everyone is screaming "witch" at one another, convinced that if they drown out all the evil, only good will remain and all the bad things will stop. So if Mary Warren is right, and there are no witches, then their hopes for peace are shattered, and with something that ground-breaking for their already paranoid society, I would hesitate too.