I wondered why it happens, too. I found
this article on hating whistleblowers, and here are some quotes that really resonated with my own experiences of backlash for summoning the courage to speak up:
[Anita] Hill, like many women and men who report a wrongdoing, whether it's done to them or to someone else, was not praised for bringing the problem to public awareness. Instead, she was attacked and vilified. Her entire personality and personal history became a target of her critics.
When someone reports a wrongdoing, we often start to look for what might be wrong with them or what might be false about their story. Why? Why do we look to criticize someone who reports when something bad is happening?
[...]
At a conference on "Speaking Out," Dr David Morgan, a Psychoanalyst for Whistleblowers UK, who has worked with more than 200 whistleblowers (that is, people who make a public disclosure of corruption or wrongdoing ) from a variety of industries, says that whistleblowers make us aware of a reality of society that we don't want to know about. "Most of us turn a blind eye. It's a good way of surviving, you get to keep your job [or place in a community], you might feel guilty but you can forget about it," he said.
[...]
But there are other complex psychological dynamics going on when someone tells on someone else, as well. One factor is that, on top of being forced to see something we don't want to see, a whistleblower can stir up our own guilt for misbehaving. So we flip the picture, getting angry at the person who represents our conscience instead of accepting that guilt. In psychological terms, this response is sometimes called projective identification– we see in the other person something we don't like in ourselves and get angry with them instead of with ourselves.
And then there's the sense (false, according to Dr. Morgan) that tattletales are often smug. They act like they are better than us; so naturally, we want to make them feel bad too. So we attack their credibility, which feels like we are protecting our own credibility.
[...]
And we attack that person, looking not just for cracks in their story, but for all of the reasons we should not believe them. Dr. Morgan says that despite the common belief that whistleblowers are narcissistic or self-aggrandizing, most simply see themselves as doing what they are supposed to do, what is right and proper, and are surprised and deeply distressed by the amount of hostility that is turned on them.