I'm not sure of a short answer that can satisfy such an inquiry and longer ones would seem to miss the mark in all possible cases of trafficking. But I'll try one avenue. Sex trafficking being one of the branches of human trafficking, I think government not making that a legal market (legal prostitution), makes it responsible for underground nature that the business operatives go with. I don't think this necessarily speaks to 'how are they responsible,' but again, not sure of a short answer that can be provided to that. Lots of things go into human sexuality and attraction, and some of that is based on have and have not type logic, or even is filtered through idea of people being 'on the market' available. If a have not is wanting to participate in the market, by obtaining a good, whatever the cost, however that occurs, they will. I see in that individual situation, the individual being directly responsible, but also the options being greatly limited by government given how illegal prostitution is. So, the any means necessary takes over, and since a market does exist (albeit underground), humans are needed to be trafficked to satisfy the individual.