People don't prefer ornamentation because they want to show off, signal social status or genetic fitness, etc. Showing the inadequacies of that line of thinking would be a post in itself. Suffice it to say that the only human case of ornamentation, whether genetic or cultural, that looks like the peacock's tail is head hair.
So then why do some people prefer more ornamentation than others? I mean that as it applies in any domain, since the preference for it cuts across all of them. If you like Art Deco buildings, you probably also like New Wave music, nicknames (particularly non-standard ones), Metaphysical poetry conceits, etc.
It is because people differ in their desire for things to be memorable. I think this stems from their social-emotional attachment styles, where the secure and anxious-preoccupied people want to hold on to memories, as they help to cement the social bonds that these people crave. The dismissive-avoidant and fearful-avoidant people want to block them out, minimize their importance, smudge out the defining features into generic stereotypes, etc., in order to keep their connections to others sufficiently loose. That does come across in interviews where they are asked to recall earlier parts of their life.
Ornaments serve as landmarks as your unconscious mind navigates its way through all of the information it keeps on file. Once you hear the opening riff of "Satisfaction" by the Rolling Stones, you effortlessly retrieve it and everything associated with it from memory. It doesn't really contribute to the melody -- it's there to make sure the song has something unique to it, keeping it from being just another pop song interchangeable with all others.
The riff is also repeated over and over to make sure it sticks in your mind, in just the same way that most ornamental flourishes on buildings or pottery are repeated in chains or grids, groupings of the same theme in different sizes, and so on. The fact that ornamentation and repetition are employed so closely together tells us that they're almost surely for the same purpose, which for repetition is obviously to aid memory. Poetry that abhors ornament also tends to shun alliteration, rhyme, and parallel grammatical structures.
Needless to say, I don't consider things encrusted with detail to be "ornamental," as the information overload hinders rather than helps you to remember it. You may not even recognize what the basic thing is underneath. Think of a singer's embellishments that obscure the underlying melody, decorative sculpture done in such high relief that the surface of the building gets lost, or a metaphor so overwrought that you forget what the point of the poem or passage was.
With this view in mind, we can gain insight into at least four areas where ornamentation levels differ, aside from the case of people of different attachment styles mentioned earlier.
First, why is the culture made in rising-crime times more ornamental? Because when primed with thoughts about their own mortality, people are more desiring of symbolic immortality. On the creators' side, writing a catchy riff or designing catchy details for a building give them a shot at outliving their earthly existence. On the audience's side, they feel like the more ornamental nature of the culture they've identified with will make it more likely to endure after their own death. They've become part of something larger and longer-lasting than their individual bodies.
Second, related to that, why do falling-crime times produce both more minimalist and more overblown culture? The Palladian and more Baroque or Rococo ideals for architecture co-existed in time, the mid-1930s through the '50s saw the rise of both drowsier pop music alongside the cacophonous Big Band swing music, and the Victorian era was home to less sublime poets like Robert Browning as well as the emo Symbolists. Both are ways of making and identifying with less memorable culture, whether through underwhelming or overwhelming means.
Third, why do societies differ in ornamentation preferences? It's a combination of needing to remember individuals, plus a large social scale, so that ornaments beyond the range of basic human body plans will be necessary to keep track of everyone.
- Hunter-gatherers live in very small groups, so they can probably get by with faces, names, and voices. Their visual culture is always drab -- plain clothing and fairly undecorated technology.
- Agriculturalists don't interact face-to-face with many people, outside their family. Just go out to the same plot of crop land, dig up weeds, sow seeds, cut down plants, ad nauseam, day in and day out. They also look pretty drab and minimalist, East Asians being the extreme case. Only the very wealthy in a rich, stratified agrarian society prefer ornament, but then they don't work the fields in isolation, but rather live a highly social life at court, where they need to keep track of who's favoring and who's slighting who else.
- Horticulturalists are highly decorated, but usually more at a group level than an individual level. Their ornaments look more like ethnic markers than an embellishment to make an individual uniquely memorable. Most of their need to remember who's benefited and who's harmed who else is across groups. The yellow-feathered tribe gave us a feast last month, so us red-feathered people will have to feast them this month. Some bunch of yellow-feathered guys raided us and ran off with some women, so us red-feathered guys will have to raid them in return. So ornament will only need to distinguish the large groups that interact, i.e. become ethnic markers.
- My favorite group the pastoralists are of course the most fascinated with individual ornamentation. See this picture of Maasai boys being initiated into manhood, and notice how individual the decoration is. (Compare to this picture of the horticulturalist Huli from highland New Guinea, whose ornamentation as mentioned above is much more uniform.) The profusion of ornamental motifs from the northwestern part of India through the Middle East and up into agro-pastoralist Europe is also well known. So is the penchant for musical ornamentation.
Pastoralists live in largish populations, so unlike hunter-gatherers they need cultural ways to distinguish individuals. Like hunter-gatherers, though, they are nomadic and interact face-to-face with others in the area. Not being sedentary, they lack the strong culture of law that comes with agriculture and even horticulture somewhat. Rewards and punishments are given out by the affected parties themselves, so they are obsessed with reciprocity, and that requires a good memory for who's who. Who was a gracious host to me last year? I'd better make sure to host them well this year. Who tried to rustle my cattle the other month? I'd better make sure to raid him back.
And fourth, this may even help explain why some groups have more ornaments genetically, such as lighter eye and hair color. No hunter-gatherers do, whether they're in tropical, highland, or Cape parts of Africa, Alaska, etc. Neither do agriculturalists, whether from Africa, East Asia or the descendants of the Aztecs and Incas. Horticulturalists don't either, again whether from Southeastern Asia and the South Pacific, the Americas, or Africa.
The prevalence of light eyes and hair across the world is just about the same as the ability to digest lactose in adulthood, suggesting a link to pastoralism somehow. There are exceptions: East Africans and Mongolians (and maybe Tibetans?) can drink milk but don't have light eyes and hair. Maybe the genes for lighter eyes and hair haven't had the thousands of years to become common like they have elsewhere in older animal-herding societies. Perhaps in 3000 years the Ethiopians will have green eyes and dark blond hair too. (And you thought they produced a lot of supermodels now...)
The idea is that lighter hair and eyes would make a person more memorable when darker colors are more common. So having light hair and/or eyes is a way of signaling your good faith to the rest of the group, like you won't be able to exploit them and disappear into the crowd like a Chinese person could in China. In a group that is hyper-sensitive about reciprocity and keeping track of individuals, that sign of good faith might get you a slight fitness advantage, i.e. by being more easily welcomed into the group and enjoying the benefits of membership. Individuals with less distinctive features would be kept more at a distance: they're harder to track down if they harm you, and they can more easily lie about having benefited you in the past -- "Don't you remember me? I was that dark-eyed, dark-haired guy who..."
Sure, this last one is a real stretch, but then it's no crazier than the other theories for why pastoralists are genetically programmed to vary more in color.