A few reasons:

This aesthetic was underpinned by a social system that has since disappeared. There are two critical components to this system.

First, during the early 20th century, the ruling class and bourgeoisie held undue sway over cultural matters. This is why the Duke of Windsor was the most influential menswear figure of his day. The photos you posted below show the intermediary period — the mid-20th century, when things started to transition. The labor movement, Civil Rights movement, second-wave feminism, anti-war protests, and general countercultural sentiments began to chip away at this authority. These were not aesthetic movements, per se, but they helped erode the old social hierarchy and, with it, the ability of elites to dictate what counted as respectable culture.

By the time Watergate happened, the suit lost much of its cultural prestige. It remained the uniform of institutional power (e.g., Washington politicians, Wall Street bankers, etc), but that was the problem. Many people no longer wanted to look like the ruling class or the bourgeoisie; they wanted to look like rebels, hippies, musicians, artists, blue-collar workers, and the like.

The second important dimension is social pressure. In the early 20th century, if you didn't follow the cultural rules set by elites, people would treat this as evidence of poor character (i.e., the politics of respectability in dress). During this period, men's dress was governed by time, place, and occasion, such that if you had to do a certain thing at a particular time, you generally knew what you were supposed to wear. People then tied those dress codes to morality. If you didn't follow these rules, there could be social consequences, including a loss of economic opportunities or social shunning. Thankfully, those codes no longer carry the same coercive force.

To me, this is the biggest reason men don't dress like this anymore: we live in a freer, more egalitarian society (thankfully). But there are others.

For instance, elegance was an important organizing idea during the first half of the 20th century. But in the postwar period, it was increasingly seen as fusty, so people replaced it with "cool." Since then, elegance hasn't been very relevant. Even people who wear elegant clothes wish to subvert them in some way, so they look cool. Today, dressing too elegantly can even mark you as an oddball.

Additionally, as demand for the suit waned in the postwar period, the commercial structures that once supported it crumbled. In the early 20th century, a man could walk to his nearest clothier and be advised on what he needed to wear. If he told his sales associate that he needed to attend a summer wedding, the associate might furnish a tan poplin suit with a pale blue voile cotton shirt and a pair of medium brown derbies. If he needed something for dinner at a nice restaurant, perhaps the associate would pull out an olive wool-mohair suit and a pair of black calfskin oxfords.

Today, such stores don't really exist. The wide array of tailoring — organized by season, time of day, occasion, and such — has been mostly reduced to four-season wools in sober, conservative colors like navy or gray (what people still wear to weddings, funerals, and court). You can get some of this variety today through made-to-measure tailoring. But made-to-measure sales associates often bait people into making a purchase by telling them that they can get a contrast buttonhole and a wacky lining. The suit can be made slimmer and shorter to align with current trends. The result is something very ugly because the customer doesn't know better and the sales associate has little interest in guiding him (assuming they even know how to produce this look in the first place).

If we can grade one’s effort from one to ten, we can say that a man in the mid-20th century needed to put in an effort level of five to make himself look good in such attire. Today, such a man would need to put in an effort level of nine because there are no good stores around him. He also needs to learn about all these arcane rules rather than being led by an informed sales associate. All the effort that was previously done by a clothier — advising the man on tasteful combinations and making sure the suit fits — now has to be done by the consumer.

Lastly, I believe men's relationship with dress remains policed by regressive gender norms, including the longstanding association between conspicuous interest in fashion and femininity/ homosexuality. For centuries, Western society has looked down on dress, considering it too mercantile and vain. Social roles during this period were heavily gendered, with only men permitted to pursue "higher" fields, such as law, academia, and politics. As gender roles hardened, a "serious" life was coded as masculine, while fashion, adornment, and conspicuous concern with appearance were increasingly coded as feminine. Traces of that bias remain with us today.

Thankfully, Gen Z is breaking down some of these norms. I often see young men who are not at all ashamed of their interest in fashion. Still, I think it's true that if a man shows too much interest in clothes, he can be suspected of being feminine or gay. Men today are comfortable dressing down because it lets them say they don't care about appearance, even if that indifference is performative. They may even put a lot of effort into looking cool. But dressing elegantly crosses that boundary, making denial considerably harder.

To me, those are the big reasons why men don't dress like this anymore. IMO, the only way to go back to this aesthetic is to make society even more liberal. Today, cosmopolitan cities remain the easiest places to wear a tailored jacket because no one cares what you're wearing. I've seen people in tutus and combat boots in San Francisco — which I think is great. When people so thoroughly smash dress rules, it makes it easier for guys who are into this aesthetic to dress however they want. In a small town, a man wearing an olive poplin suit in the period-correct Ivy Style cut with a black knit tie might be considered an oddball; in a big city, he’s considered “normal.”