Police who would never do certain things in their own country can treat those same things as normal once they are posted far away. Outside their usual surroundings, with colleagues around them acting as if it is all fine, the abuse gets folded into ordinary life. In the book's case, brothels were dressed up as restaurants and dance clubs, and the foreign clients came and went openly.
Ethics courses and signed promises to avoid prostitution and trafficking are lip service when the people signing don't understand what is at stake. Nobody checks on them afterward and nobody is held to account. Contractors also sent staff abroad untrained in local law, international law, or the cultures they would meet.
An investigator may first see a few officers taking bribes to look away, then officers buying teenage girls, and finally peacekeepers running the trafficking themselves. Each layer makes the earlier explanation look too small. A minority of about one in ten can corrupt a mission where most people do good work.
When the wrongdoers can't be prosecuted and exposure would threaten lucrative contracts, the institution's interest shifts from stopping the abuse to burying it. The worst offenders are then sent home rather than charged. Meanwhile the company keeps winning new business.
An organisation protecting itself first attacks the person raising the alarm, calling them burnt out, before it ever looks at the evidence. Demotion and dismissal follow. This leaves the whistleblower working alone and unable to trust their own colleagues.