The rich, they are different
There has to be something very satisfying about killing off a rich person. The mystery genre’s very foundations rests on the corpses of the well-to-do; that any hereditary titles survived the Golden Age is astonishing. Those plump inheritances, the isolated country houses and silently judgmental domestic staff, often coupled with a victim and cadre of suspects that usually don’t generate too much sympathy (they did have a pretty comfy existence before the fatally poisoned claret, after all)—the mystery basically writes itself.