Mostly because people don't think things through before they make them illegal.
Prostitution has never been a reputable business. We connect sex and love and to make sex about money FEELS wrong. It FEELS dishonorable. However, the women involved keep selling it, and the men involved keep buying it.
So making it illegal hurts everyone. The hooker is less safe because if her 'john' is violent she can't go to the cops for protection nor can she operate in a respectable place where she can be protected. So she has to work for/with a pimp. The pimp just siphons money away from her and often abuses her, but it's better than being killed by a john.
Meanwhile the john is also unable to be sure he's going to someone who is safe or clean, he is vulnerable to the pimp, and similarly can't call the cops if defrauded.
There's more disease, more crime, more disrepute.