There's this common meme of blaming anti-sex-work sentiment on some bloc of US Evangelicals who secretly rule the Western economy and politics. Because surely, it couldn't be
the market speaking out against adult material?
Here's an alternative hypothesis: between those anti-sex fundamentalist religious groups (Christian or otherwise) and people who absolutely love everything sex-related, there's the vast majority of people who find the whole topic uncomfortable to a large degree. Whether they themselves like porn or not (or like, but feel guilty about it[0]), whether they make use of various meatspace sex services or not, they don't like being reminded of it. They don't like sex being mixed with other areas of life. They might especially not want their kids to be exposed to porn, or to become sex workers.
The market caters to majority case. Advertisers, especially, cater to majority. In such hypothetical world (which I posit might just be the real one), the market itself will exert pressure against overt pornography and sex work, and anything that associates itself with it.
So, perhaps, there's no conspiracy in play - maybe it's just the majority getting the things to how they want them to be.
(It's supposed to be good, right? When some people here complain that technology is increasingly user-hostile and exploitative, there's no shortage of arguments saying it's not conspiracy, it's just the market catering for majority use case. So which way is it?)
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EDIT: Three more thoughts.
1) Of course, the degree of discomfort with sex topics is a spectrum. But on this spectrum, sex work and hardcore pornography are one of the extremes, not the middle point. Thus, the majority being uncomfortable shouldn't be a surprising concept.
2) I suspect the same thing happens with drugs. There's people who are strongly anti-drug, often on religious basis. Then there's people who are for everyone (responsibly) ingesting whatever they want. But in between, there is - in my experience - the majority who's afraid. Afraid of health effects, afraid of associations with organized crime, afraid of being shunned by others. And so, even legally selling mind-altering substances (other than alcohol and cigarettes, which have been normalized over centuries) faces the same challenges as sex businesses.
3) One can say it's religion all way down. Perhaps. Atheism as a cultural phenomenon is a very new thing in the history of humanity. The way people think about sexuality carries thousands of years of baggage. It's not going to change overnight, or over investment round, or over election cycle. So I don't think you can fix the problems faced by sex workers by telling Visa or Mastercard to chill out.
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[0] - There's no shortage of pundits - secular, religious, and religious pretending to be scientific - that make careers out of convincing people that porn is bad for you.