"The Purloined Letter" is not as intriguing as Poe's other detective story, "The Murders in the Rue Morgue." The problem with the story about the stolen letter is that it requires a great deal of exposition at the beginning and then a great deal of explanation at the end. After Dupin turns the letter over to the Prefect and collects the reward, there are, in my edition, nearly eight pages in which Dupin is explaining how he deduced the manner in which Minister D- had managed to conceal the letter in plain sight and how he got it back from him.
This sort of "post mortem" explanation is common in mystery stories even today, but it is not dramatic and is customarily gotten over with as quickly as possible. One mystery writer quoted a mystery magazine editor as saying, "No explanation without heat!" What that meant was that when it became necessary to explain the solution to the mystery, the bad guy ought to be pointing a gun at the detective, or holding the detective's girlfriend in front of him and threatening to cut her throat, or even waving a hand grenade and threatening to blow up the building. These hackneyed devices and many others were invented in an attempt to keep the story dramatic while someone is giving a long lecture about how he solved the crime.
In many of Agatha Christie's novels about Hercule Poirot, the fabulously successful English author typically has the detective explain his deductions and actions but hold back the identity of the killer until he had completely finished with the obligatory lecture on his solution. The reader is kept in suspense waiting for the revelation of the murderer's identity (usually the character least suspected).
Poe didn't write any more Dupin stories after "The Purloined Letter." He may have felt dissatisfied with that story but did not want to write pure thrillers in which the emphasis was more on character and action than on the ratiocination which he loved so dearly.