Why is human trafficking difficult to stop?

Let's start with understanding the term: human trafficking means trade in humans often for the purpose of sexual exploitation or, forced labor. In 2005 global annual profit from human trafficking was estimated for $31.6bn.

I believe that human trafficking is so difficult to stop mostly because of its profitability for traffickers, brothel owners, madams etc. Another factor is social alienation and stigmatisation of victims by society. Governments often do not offer enough support for trafficked victims. Finally, the victims are under huge psychological pressure which is often hard to handle and leads to consequences like learned helplessness or Stockholm syndrome. 

It might be the second one