Melinda stays silent due to trauma and fear. The summer before entering high school, Melinda was sexually assaulted at a house party. She ends up calling the police which causes her peers to hate her for being a "snitch." However, they are unaware of the real reason she calls which completely isolates her.
This event is deeply traumatic, and it causes internal and external conflicts. Internally, Melinda is dealing with pain, confusion, self-loathing, and the fear of the incident repeating. The shock causes her to repress the event creating flashbacks and other symptoms of trauma. She ends up living in survival mode for most of the story which causes her to remain silent so as to not rock the boat at school or home.
Externally, her world is also a mess. Melinda's friends have abandoned her, and the school knows her as the girl who called the cops and ruined the party. Who is going to believe her, even if she does muster the courage to speak up? She is too afraid to deal with her trauma, and she doesn't want to reach out to anyone fearing they won't understand or won't believe her. Melinda's internal and external realities keep her in the trauma cycle.
While it may seem odd that she stays silent, it speaks to the very real experience of sexual assault many people face. Trauma and repression make it difficult for the victim to understand what happened, and the fear of consequence reinforces their silence. Melinda follows this pattern up until she finds a small support group that gives her the courage to stand up for herself.