Human trafficking

Human trafficking is the act of gathering, moving, receiving, or keeping human beings by threat, force, coercion, or deception, for exploitative purposes. This includes “the exploitation of prostitution of other or other forms of sexual exploitation, forced labor or services, slavery or practices similar to slavery, servitude or the removal of organs.” This definition has only been in time to time from now. Why does this things always happening? Why this things happens to those people who is poor? Why?