Lots of reasons. I was molested as a kid. At first, I felt flattered that my teenage uncle wanted to spend time with me. I was 6 when it started. I actually had a crush on him (which isn't unusual when kids don't understand the whole concept of incest). Later, when it progressed to sexual contact, I felt uncomfortable but I couldn't articulate why. I didn't even know what sex was -- so I didn't have the vocabulary to discuss it or the understanding of sex to know why it was wrong. When I started to object was when the threats started. I was told that my father wouldn't love me if he found out what I did. I was deeply ashamed. I thought that I had done something very wrong. As I got older, the reasons for keeping quiet changed. I didn't want anyone to find out that I was the girl with the uncle who molested her. I feared what my peers would think and say. I also didn't want to hurt my grandparents because I knew they would blame themselves since it happened when I was sleeping over at their place. I didn't want my father to go to jail because I feared he would kill my uncle if he found out about it. I kept quiet until I started therapy when I was in college. So, it really isn't a surprise that kids don't tell anyone.

And, for the record, some of them did say something and they were told that he was doing legitimate medical procedures. They weren't believed and nothing was done to stop him and he kept abusing girls as a result.

because they were children