And because women involved have no police protection as long as this is illegal. They will always be the most vulnerable members of society afraid to get help.
The other side of this is that victims of sex trafficking are often so emotionally attached to their pimp that arrest is sometimes the only way to separate them and force them to talk to a counselor. These women are often mentally impaired or were very young when they began "dating" (working for) their pimp. Convincing them to not just leave but testify against the person clothing, feeding, and sheltering them is a tall task.