Something curious about the ITV version of Poirot is that (with the exception of The Mysterious Affair at Styles) all of its episodes were set in the 1920s. Not literally the 1920s, per se; I’m sure that plenty of the technology or fashions were from the 1930s, but neither the Great Depression nor the looming war due to the military buildup of Germany ever feature.
This is not true at all of the novels.
The Poirot novels are always set contemporaneously to when they were written and current events, or at least current conditions, play into the plot. The only anachronism is Poirot himself; when Agatha Christie first wrote him, she presented him as being at least in his sixties. In her autobiography she mentioned that this was an unfortunate choice on her part, but she had no idea how popular he would be or how long he would last, and as of the time of her writing about it he had to have been over 100 by then. She simply ignored this problem and made Poirot always an old man of unspecified age.
When ITV made its version of the stories with David Suchet, they chose to set all of the stories in the same few years, though rarely with anything that would date them. There were practical reasons for this, of course. For example, it would be difficult to age the actors appropriately by decades in order to follow the real stories. Wardrobe and set decoration would be far more difficult if they kept track of the changing styles. Moreover, a series of episodes (or short movies) would be far more jarring if they skipped forward by years every few weeks or months, while the books always skipped ahead by however long it had been since the last one.
However many practical reasons to set Poirot in the span of a few years, though, I suspect that the biggest reason was that the 1920s are simply far more interesting, and far prettier, than later decades. This isn’t the totality of the 1920s, of course. Poirot was a celebrity and tended to deal with clients of means. Accordingly, the stories are set largely among the prettier parts of the 1920s. This is as it should be. Detective stories are stories for the common man, and so they should deal with things that he will not normally come across. Fiction about the lower classes is the domain of the upper classes, who need to read about drudgery and difficulty to find variety from their lives.
There are complex reasons why this should be, but the one thing I think it isn’t is rose-colored glasses from anyone’s past. By the 1990s when ITV was making the Agatha Christie’s Poirot series, the 1920s were seventy years before. No one remembered them. Instead, if we look to the specifics, we will find a decent answer. The 1930s were an interesting time but heavily influenced by the world-wide Great Depression and in the later portion by the looming war on the European continent. The 1940s were dominated by the second World War, to the point where no one ever talks about the events of 1946-1950. The 1950s had a primarily industrial aesthetic, as people took refuge in the post-war plenty which was so different from the great depression and the war years. In more rarified circles, architects and designers were greatly attracted to anything which was not beautiful. This was the era of the Helvetica font and the beginning of the era of buildings which no one likes. The 1960s spiraled off into kaleidescopic colors that meant nothing but were fun and new. The 1970s were, of course, varied, but let us leave it with two words: shag carpet. That takes us to the end of when Poirot stories were written, but for completeness: the 1980s were the era of big shoulder pads and bigger hair with leather jackets and denim jackets, while the 1990s… I wonder what the style of the 1990s even was? T-shirts and jeans or shorts? It’s been thirty years since 1993, and has anyone figured out anything to be nostalgic for? Classic video games are the only thing that I can think of.
Anyway, I think that I’ve made the point. The 1920s are an era with a fascinating aesthetic that’s pleasing to look at, and it was the last time to have that for quite some time. (Portions of the 1930s were more-or-less continuous with the 1920s, but I’m counting them as part of it since they were, aesthetically, a continuation of them.) There will be others, of course. At some point our fascination with trying to see how little clothing people can wear will be over, and people will try to make their clothing interesting rather than revealing, again.
This is not the same thing as nostalgia for the 1920s, by the way. I don’t think that it being fun for Poirot to be set in the 1920s is nearly the same thing as wishing to live in the 1920s. It’s merely a recognition that the interesting parts of the 1920s were very interesting, while the interesting parts of later decades weren’t nearly so interesting.
There is also the argument to be made that the 1920s (and 30s) were the last real era of the private detective. After World War 2 we live much more in the era of the spy thriller. In the spy thriller people kill and are killed for governments and large organizations; we don’t care nearly so much for the concerns of the individual. There may be some truth to this, though for all that people still go on murdering people for their own reasons even in the 2020s, and people even still care when people are murdered. It may be fewer than in former times, but detective stories were always about unusual people.