What other American cultural characteristics or traditions are at play in the genre?

Hard-boiled fiction is a sub-genre of detective fiction that draws on numerous literary traditions, both US and European.
The detective story, featuring a singular sleuth ingeniously following clues, owes much to nineteenth-century pioneers, notably Edgar Allan Poe and Arthur Conan Doyle. Skillful detectives also star in Agatha Christie's works, where the Belgian Hercule Poirot applies his talents. Christie's Miss Jane Marple represents a departure, as she is an elderly lady. For elaborate trails of clues replete with red herrings, Dorothy Sayers's eccentric English nobleman Lord Peter Wimsey is hard to top.
But in the US, especially California post World War II, ordinary guys also take center stage. While Dashiell Hammett's Nick and Nora Charles are well-off, Sam Spade is street-smart and tough. Ernest Hemingway's plain-spoken macho heroes are one antecedent.
Looking back, American realism overall and western, cowboy heroes established the stoical archetype. The so-callled dime novels and magazines so popular in the early twentieth century helped create a broader reading public who wanted action and relatable heroes, not literary fiction. Raymond Chandler, creator of the quintessentially hard-boiled Philip Marlowe, explicitly rejected the artifice in Sayers's plotting and Wimsey's elite indolence. His treatise "The Simple Art of Murder" is an insightful look at what makes the genre special.
https://books.google.com/books/about/The_Simple_Art_of_Murder.html?id=z40A6qE9tucC&printsec=frontcover&source=kp_read_button

https://www.britannica.com/art/hard-boiled-fiction