Why does Marianne refuse to testify against Zachary in regard to her rape in the novel, We Were the Mulvaneys?

After the rape, Marianne is ashamed and does not tell anyone until her mother drags her to the doctor's office and forces her to tell. Then, Marianne refuses to testify against the boy, because the alcohol has so clouded her memory of the event that she is afraid she will not be able to testify to the truth.