I believe it is still mostly the influence of religion on the over all culture. Remember religious belief was at one time mandatory in Western culture. Nonbelievers were often tortured and put to death.

Personally, I've gotten past the religious conditioning and have no hang ups about nudity. It was a matter ofr unlearning what I was taught as a child.

Lack of interest on my part.
Dysphoria doesn’t help.

Education.

this site is prudish also

How about a sense of what is beauty, and what is not?

I tend to think that the human disgust/prudishness towards nudity stems directly from the bible itself.
That being derived from the Book of Genesis where the entirely mythological Adam and Eve suddenly learned and discovered that they were completely naked and hence decided to wear leaves to cover up their 'private parts.'
Early humans would have used animal skins as a kind of covering to keep warm in the cold months and climes most likely and probably gone at least naked or partial naked in the warmer months/climes but when Christianity and earlier belief systems began to take hold on societies then came about the necessity for clothing the body.
It was not until Europeans invaded ( settled) in Australia, for example, the the Indigenous natives began wearing any types of clothing nor felt shame, etc, in being completely naked except for on and during ceremonial occasions.
After all, are we NOT all born completely naked?

Jealous men not wanting other men to look at there women?

Probably still religious in some ways. Plenty of people try to make it about some objective moral imperative something-or-other, but in reality it's their religious upbringing talking.

But considering, for reals, for reals, the best reasons I can conceive in terms non-religiosity, there's a lot of little reasons here and there that I have sussed out in my travels.
In the Mexican culture, for instance, the idea of homosexuality is frowned upon almost viscerally. There seems to be a kind if idea that it's unmanly to even validate homosexuality as a legitimate practice or behavior, or to even talk about it in a "philosophical" manner, if that makes any sense. Even then, it's an inadequate reason by itself, because the responses are so reactionary. It's not everybody, obviously, but the more traditional Mexicans or the "cholos" have an aversion to discussing and even joking about homosexuality. It leaves a bad taste in their mouths of unmasculinity or something.

For a lot of women, I think they feel like behaving in a prudish way is a defense or deterrent against anyone who would make sexual advances. This is all fine and dandy on paper, but I have been around plenty of women - who, mind you, I had no intention of making any sexual advances on - and they react, uh, poorly to jokes or even adult discussions about sexuality. And again, it's not like they wake up in the morning and say, "I am looking to deter potential predators." Their motivations are more nuanced and ultimately more arbitrary and reactionary than that. And again, not every woman is like this. I've seen men behave in this manner, too.
Even so, it's tough to 100% say they're in the wrong in doing this, because there are a lot of dudes who would be given that inch and take the mile, so to speak. I've taken that inch to a certain extent, too.
And also, it's one thing for me to joke and she laughs, but then it's a whole other thing if she starts joking back or something. Then I might think she's flirting. But then it's like: Is she casually flirting and/or making her own jokes or is she dropping hints? At that point, I've always been a person who openly communicates these thoughts and it has turned some people off, but it's helpful for me to dial back my fuckboy parameters when I directly ask, "Are you into me?" "Are you just playfully flirting or are we feeling each otherrrr...?" "I find you attractive, do you feel the same about me?" Not that I find myself in those situations often. I don't like to read into signals or play games or beat around the bush, or basically make any moves that aren't sanctioned, but it's how a good chunk of people do these things.
So, overall, what I'm trying to say is that some women find it best to react in those ways so as to not be misinterpreted at all.

However, some people might behave like this, all prudish, but secretly love the sex talk or jokes or advances. I never "invest" in that idea, you know. If someone is acting prudish, then I assume they really are prudish and they can walk away from the conversation or they can sit there quietly or I stop talking to them, etc.

I think a lot of it, also, has to do with training and upbringing. Somehow, they are taught: sex=bad (usually through religious context). Some people internalize it, and again, it's not like they wake up in the morning with their motivations and rationale fully loaded, but their behavior comes from a nebulous version of those things, tempered through their upbringing. It manifests as a reactionary disgust for the topic.

So, I'm talking about prudishness in a certain context. I'm using the highest level of prudishness which is inversely proportional to the "intensity" of sexual expression. A light masturbation joke about myself in a group setting might lead someone to react by insulting me saying, "Keep that stuff to yourself, you moron!" This has happened before. This is, for sure, the most unacceptable form of prudishness to me. No one is directing the joke or comment at you and you shouldn't impose this particular moral view on others, especially when everyone else is okay with the joke.
Now, as someone else pointed out, showing someone a dick pic and them reacting "poorly" can be considered prudish. I think in a very technical sense it is. If you take only one situation with a dick pic, and the other person blocks you, then it's prudish. But if one doesn't realize the weight of 100 dick pics, that's all that the one situation will seem - a "poor" reaction.
I kind of like it when someone sends me an unsolicited pussy pic, it makes me feel "wanted beyond their control" in a weird way. Not that I get those often, either. But how would I feel if I was sent dozens of pussy pics a day and treated like I was only good for my penis? And the reverse: would women feel unwanted if they never ever had received dick pics?
Any ladies want to help me answer this question, DM me?

All in all, I think it stems from the very fact of this cultural appraisal: sex is a different beast as a topic of discussion. And ultimately, the worst form of prudishness comes from the vague idea that "You just shouldn't talk about it." The most "allowable" form comes from trying to protect oneself or even others. I had a lot more to say, but I got tired of writing and it feels pretty rambly🥱‍

It would be different for men and women I think, especially if a woman has children.

Definitely spill over from religion. Ruins of the old Roman baths and toilets tell us it wasn’t too long ago when we let it all hang out. Modesty means no one sees your ‘naughty bits’ or else sky people cry.

Shyness?

It's possible that variation in sexual tastes helps reduce mate-competition in large human societies; a sort of sexual niche partitioning.

Criticism I suppose...

There is a cultural aspect we accept naked children but after a certain age we expect them to be wearing clothes. There is the social norms we all face you would not go into work in an office with a see through shirt that you might wear to a nightclub. Even makeup etc we are all about projecting an image to make people see us in a particular way ie at work efficient focused on the task, at a nightclub, fun loving, available (to the right person) sexy etc. In one episode of Sherlock he meets Irene Adler and she fusses over how to dress to meet him, she greets him naked and he can not get any clues about her. There is nothing for him to "read" As for me well with someone I trust I am fine but I am not going down to a nudist beach anytime soon I don't like my body enough nor am I confident enough that others would not judge me. Sexuality what people do is up to them as long as it does not involve children or animals. (Children can not give consent as they do not fully understand what they are giving consent to and I am a speciest when it comes to sexual activity) I also dislike relationships where one person has all of the power and control. I am not talking about relationships where one person willingly gives up that to the other but ones where socio economic differences, educational differences, even citizenship status are used to make the other person toe the line. I grew up watching many "mail order brides" as they were referred to come over from Asian countries, their families were poor, they had no education and they married men over twice their age and rarely managed to learn English, (most of the men were European and they sometimes learned his native tongue) many were kept isolated through having children and no support network to help them learn English and to develop skills. Some always looked scared and I am sure a few were beaten if they did not do what their husbands wanted or to his standard.

It has to do with religion.