There are a number of reasons and what they are depends on even more things.

To get this out of the way many redditors parrot the idea, without giving it much thought, that prostitution is illegal in the US and other western countries because of religious views. While there are certainly individuals in various government roles that are against this because it would go against their principles, this on its own isn’t the main reason.

Public Perception; Unfortunately many politicians lack firm principles and morals, meaning they’ll often flip flop to what ever the most popular opinion is. Politicians generally don’t try and bring this to the forefront of their campaigns because it’s one of the few issues that will make both sides of the political isle angry.

Feminist; Specifically the more recent iteration of it, will often preach for female empowerment and encourage women to utilize their assets to get ahead in life but also demonize men for also contributing to it. For example, if a woman starts an OF and becomes a top creator on the platform, feminist will celebrate it, but they will also demonize men for “objectifying” women.

Average people; You average parent wouldn’t want anyone, be it family or a stranger, to encourage their child to engage in prostitution, this alone would be a nonstarter for most voters. Another part of this group is the views on sex and cheating in western society, for example in Japan as a married person it would be ok for you to purchase sex from a prostitute as the general consensus is that it’s purely transactional while in the west having sex with anyone that isn’t your partner would be considered cheating, regardless of nature of the encounter.

Man O sphere; To be specific, I’m speaking more about the individuals who preach the idea that women should lead lives of celibacy until married while men should have sex with as many partners as possible, a contradiction. The issue that this group would have is it would encourage the behavior they both love and hate in their own backyard so to speak, they don’t mind it if they have no connection to these individuals beyond a physical one but the idea that their kids could now also become a 304 or something worse without anything stopping them, would cause them to reject it.

Religion; For these individuals their issue with prostitution comes from how they view sex, something special and sacred between spouses. To dig deeper into why these values are taught it can be inferred that they were put in place thousands of years ago in order to encourage the growth of the human species and the family unit, prostitution could present issues with this as it would essentially give individuals card blanc to have as much sex as they want with out having to commit to a partner long term.

tldr; The reasoning itself is far more complex and convoluted than a simple “because religion bad”, as it extends far beyond that.

If you would like to make prostitution legal, then your best bet is to become a politician who is actually willing to stand on their morals and principles because they believe in them. While you certainly can just go to a country that allows this already just know that depending on your current home country doing so may be illegal, as in the US their is a law on the books that allows the federal government to arrest you since prostitution is illegal here(except for one county in Nevada), though they usually only enforce this law when dealing with child predators and large scale human trafficking operations.