Some iteration of this topic does indeed come up here with some regularity and it is one that I find very interesting (I started one of those threads some time back). I think that it may not merely be “stagnation” but rather a move away from how fashion and pop culture were treated in the 20th century (and perhaps before). Basically, while trends still exist, I think about a decade ago, we stopped having prevailing trends that were dominant to the extent of being “must-haves” or “ubiquitous”. I think the answer to why is somewhat complex and possibly due to a variety of factors. Some of these have been mentioned above, but here goes:
The Internet and other digital and electronic devices makes us independent from the offer seen on regularly scheduled television, in magazines, etc. We can browse, pick and choose at any time of day, seek out content creators that provide what interests us, and ignore those that don’t. Want a movie or TV show? Go on Netflix and choose what you want to watch. Remember when on an airplane flight they would show one movie on screens in front of you? Now you have in-flight entertainment systems that are basically Netflix in a box on the seat in front of you.
Generally, society has moved away from conformism and toward doing your own thing. This is probably reflected in people’s music choices and certainly seems to be reflected in people’s fashion choices. Look at pictures of women from the 1800s. In a given decade, they seem to all have the exact same cut/shape of dress: when crinolines are in fashion, they seem to all have them. When bustles are in, ditto. Hairstyles too tend to follow one basic model. If you look at lots of photos, paintings or engravings of women in the 1880s, it is very difficult to find one who doesn’t have this hairstyle: a bun or twist with bangs. The size of the bangs and the amount of curl in them varies; whether the bun is twisted or braided or whatever varies; almost nothing else does. Going forward through the 20th century, you see the same thing: people, especially women, tend to regularly change their look with the fashion, there is no “retro fashion”, only some tolerance of older people wearing outmoded styles from their youth. Up until perhaps the 1960s, deviating more than a little bit from the standard fashions of the time would probably have been seen as eccentric, if not borderline anti-social. Now we can go online and criticize the fashions of the time; “retro” is a thing, and “doing your own thing” is a thing. And stores - both online and brick and mortar - cater to various tastes and styles. In the city where I live, while there are certain things I can identify as a trend, currently there is nothing that I would say is a clearly “dominant” trend - I see so many styles on the street, some of them reminiscent of things that were worn in the last 4 decades - that I can only surmise that everyone is readily able to develop “their own personal style” - and that people do it.
Trends today exist, but they are fast - what is in style today may be forgotten in a few months. An alternative to this exists, which was allegedly conceived to buck the trends - the idea of “normcore” fashion. I.E. wearing generic basics like jeans, khakis, t-/sweat shirts, polos, etc., that are practical, standardized, and meant to be a reaction against fashion trends. My own style of dress is basically normcore. Those who do care about fashion will often be interested in individual influencers’ suggestions (again, we are back to the Internet) rather than what some big fashion house in Paris, Rome or New York is putting out or what some actor/actress is wearing.
Another factor: we can be said to have tried everything. So many different trends have been put out over the years that pretty much anything that comes back can be said to have done before. With the choices you have today, you can pretty much pick and choose any earlier trend that you like and wear it. I’ll bet, however, that the photographic evidence of some of the more excessive trends (e.g. the overstyled hair and excessive makeup of the 1980s) is causing some trends to stay buried. I wonder if a large percentage of people haven’t simply figured out what looks good and are sticking to it?
The factor of “rebellion” was mentioned above. A lot of fashion trends in the past, certainly in the 20th century, were indeed a result of rebellion, e.g. rock and roll, hippie fashions, zoot suits. But so many taboos, conventions and traditional values have been torn down: the sexual revolution, acceptance of LGBT people, Vietnam is ancient history; there is simply less to rebel against today.
I would also like to note that one reason why posters above have given different impressions of the matter may be the fact that fashion trends at any given moment may not be the same everywhere; some trends are more popular in one place than in another. I will give some comparative examples from Toronto (and area) and Prague, the two cities in which I have spent most of my life. Someone above mentioned young people preferring sportswear to jeans in recent times above. I noted this in the Toronto area 2-3 years ago, lots of school-age kids wearing black track clothing. A similar look is seen among teens in Prague, but seems not to be as common (or to be seen mainly on boys). Girls tend to like fashion jeans (when appropriate for the weather), as well as leggings. Boys often wear skinny jeans. Another example someone mentioned above is of unnatural hair colors being in style. In Prague, I see that very rarely nowadays (usually among middle-aged, particularly working-class women). However, when I came to Prague in the 00s, I was unpleasantly surprised (as were some other expat men) at how common unnatural hair colors were on girls here back then. You’d see lots of things like three different colors, or for example shoepolish black hair with bright red streaks, and that on girls that nature had been very kind to. However, I believe that women in general used to color their hair more in the recent past than they do today. Again, only speaking for Toronto and Prague, about 11 years ago, over a certain age it was hard to find women who didn’t at least have some streaks/highlights. At one point it seemed that around 80% if not more colored their hair. But over the course of the New Tens, the notion of embracing your natural color seems to have gained more popularity. I distinctly remember a time in 2016 or thereabouts, IIRC in both Toronto and Prague, where I seemed to see a relatively high number of women with significantly grown-out bleach. Speaking for Prague only, colored hair has been less common on teenagers and younger women probably for the last 10 years, and the average of dyed vs. natural on the whole might be something like (wild guess) 50-50. Or at least there are significantly fewer women who color than in the past.
One last observation. I think in general, the 20th century was a period of massive progress and change. A lot of things that were assumed and conventional in 1900 were ancient history in 2000 and in many cases, long before then. I think that our progress has plateaued or tapered off in some areas as we have found the “right” formula for it. For example, the latest models of aircraft are unimpressive compared to the various innovations that happened in the 40s, 50s, 60s, or even the early 1990s. Or trains in North America are largely the same today as they were in, what, the 1960s or 1970s? This doesn’t mean progress has stopped, just that we have found the formula that works for certain things and so innovation may not progress on a steep curve forever. Perhaps, with all the different styles that have been discovered and that are available, we can say the same thing of fashion?
In my opinion, rather than lamenting that our current pop culture doesn’t have any distinctive trends by which to mark our era and decade vis-a-vis the 50s, 60s, 70s, 80s, whatever, we should celebrate that we live in an era of choice, where there is more of an opportunity to select from various options and less pressure to conform to the current trends.