There are still many religious organizations out there that are opposed to prostitution (almost all of them, in fact). And many of the members of those religions see prostitution as immoral, and absolutely believe that they should be able regulate other people's morality, even when it doesn't actually affect those them or cause harm. And there are even other non-religious people who see this as a public health issue which should keep it illegal. Most are not weighing or considering both sides of the issue honestly. They believe that they are right, and that is it.