Set off by consideration of The Murder of Roger Ackroyd by Agatha Christie, it occurred to me that the popularity of the genre is a statement of love of rationality. There is nothing postmodern about this, interrogating the claims of whether murder is a social construct or there are subaltern voices questioning whether it is actually a good thing. The murder has happened and someone has done it. The police, the detective, and the other characters in the story are trying to discover a real answer to this. Not only is there no bothering about what type or class of person has done this, it is often a part of the story to specifically make fun of this, of relatives or policemen who have allowed such prejudices to overrule their intellects, leading them to suspect the wrong person.
This is also the case in the True Crime genre, for similar reasons. I discussed The Man From The Train last year, in which Bill James shows that people repeatedly overlooked or even refused to believe who the murderer was because thy had some prejudice about who it mostly likely was. See also the DC Sniper, where a high percentage of the reporters for Great Metropolitan Newspapers insisted that the shootings must have been done by a type of person they didn't like, but the guilty parties turned out to be quite different. These things infuriate us, that people could lose their rationality so easily.
No, there is an actual murderer, Colonel Mustard in the Library with the Candlestick or whatever. Even when authors have great fun showing how many people had motive, method, and opportunity to murder Miss Elsie Greene, or even tried to and falsely thought they had succeeded, there is in the end a true solution. We follow closely what the detective is doing, not because he wishes to grow vegetable marrows or can identify cigar ash, but because we know he wants to get at the truth, and so do we.
I offered the theory years ago that murder mysteries only become popular in societies where murder is less common. This seems related.
Also related, George Orwell's The Decline of the English Murder.