Robbie, I think there are several reasons CSA survivors can identify with "To Kill A Mockingbird" so much. The whole premise of the book and later movie is that something whose sole purpose is to give joy is thoughtlessly killed. When looked at from the point of view of a CSA survivor, we see the parallels with our own childhood experience - we were a gift from God to give joy, yet we had our innocence killed.
Additionally, we see in Scout, Jem, and Dill, children who are having to learn about the ugliness of the world all too soon in their young lives. Again, the parallels with our own CSA experience is obvious.