Because people lack faith in God, they are afraid of poverty and they simply don't believe that God is the only one who'd provide for them, therefore they would go into prostitution. just a bunch of lost sheeps if you ask me.
If you go to countries such as Thailand and Cambodia, you can find that most of the prostitutes are religious Buddhists. So lack of faith is not the reason for prostitution. And one of the emerging hubs for sex tourism in Asia is the Philippines, which happens to be a deeply religious Catholic country.