Many have religious objections. This is probably the largest single reason. Almost all organized religions consider prostitution to be evil or sinful. Since the majority of people in the world are at least somewhat religious, it follows that prostitution is likely to be unpopular. There are other reasons. Some see it as a way women are objectified and made into sex objects. Some people consider sex to be an intimate act and mechanical sex with a complete stranger is unappealing or disgusting to them. If people are not affected personally by the above three or other factors, they often will still at least openly condemn prostitution, because of the way they wish to be perceived by society.
Now, the case for prostitution being illegal is far weaker. The case for policing the sexual activity among consenting adults has never been good, and now, given the accessibility of birth control, the one viable argument that could be made is less valid. Policing it is a waste of the police's time and money, and the taxes that pay to police it could be put to better use. How it concerns anyone beyond the individuals involved is unclear to me, and, according to my definition of "personal liberty", a person should be able to do anything he or she wants as long as he or she doesn't infringe on any other person's liberty.