Nathan - I'll just give a simple answer here while I ponder more deeply later. My read on the familiar abuser is it takes time to "worm" their way into our trust and confidence. To accomplish this they must draw close to their prey. How can a stranger get close without suspicion but rather someone we trusted AND was trusted by our parents (unless they were perps).
Once they got our confidence and trust; once they began the abuse, it became more difficult to tell on a family member or friend. How easy would it be to 'dime' on a perfect stranger? But on a brother, sister, cousin?
On our mom or dads sisters or brothers?
On our parents mom and dad?
Who would believe us? But - a stranger - that makes it a tad easier.
Even a family friend, who was once a stranger, gains the trust of the child's care- taker...teacher, preacher, scoutmaster, etc.
I know this seems simplistic but these scenerios occur over and over ad infinitum. Beware of the 'Trojan Horse' within our own secure walls!!