"Why do some boys become victims and others aren't?"
That's unanswerable, but I'll give you my observation. Some boys (and I was one of them) feel unloved and alone. Every person in the world needs attention and affection. Because they don't feel loved by their parents or other family members, they become susceptible to predators.
There are exceptions and other reasons, but think of it this way. The boy already has a relationship with a family member or someone in the community who is in a position of trust. They might be neighbors, teachers, church leaders, politicians, or a store clerk—anyone whom the boy looks up to, admires, or trusts.
The point is that the perpetrator already has some connection. That authority figure befriends the boy, giving him needed attention. The boy feels wanted, accepted, and perhaps loved. The perpetrator has gone after the innocent boy and destroys his childhood.