According to a child abuse prevention campaign in the United Kingdom and Ireland called Stop It Now!:
There are many understandable reasons why a child victim of sexual abuse is not likely to tell anyone about their abuse. Often, the abusive adult will convince the child that they won’t be believed or that they are somehow responsible for the abuse and will be punished for it. The child may care about or feel protective of the person who sexually abused them and may feel they’d be betraying this person by telling about the sexual contact and the abuser may use this information to help maintain the secrecy. Children frequently remain silent to protect a non-abusive parent from upsetting information.
Sometimes, a child may be confused if they experienced positive physical pleasure, arousal, or emotional intimacy from the abuse. This confusion can make it difficult for the child to speak up.
A child may feel that they permitted the abuse and should have been able to stop it. Remember that there are no situations where a child is responsible for any sexual interaction with a more powerful child or adult.
People who abuse children may offer a combination of gifts or treats and threats about what will happen if the child says “no” or tells someone. They may scare the child with threats of being hurt physically, but more often the threat is about what will be lost if they tell (e.g. the family breaking up or someone going to prison).
In order to keep the abuse secret, the abuser will often play on the child's fear, embarrassment, or guilt about what is happening, perhaps convincing them that no one will believe them or that the child will be punished. Sometimes the abuser will convince the child that he or she enjoyed it and wanted it to happen.