With no explanation, label text_A→text_B with either "DON'T KNOW", "NO" or "YES".
text_A: Poirot is less active during the cases that take place at the end of his career. Beginning with "Three Act Tragedy" (1934), Christie had perfected during the inter-war years a subgenre of Poirot novel in which the detective himself spent much of the first third of the novel on the periphery of events. In novels such as "Taken at the Flood", "After the Funeral", and "Hickory Dickory Dock", he is even less in evidence, frequently passing the duties of main interviewing detective to a subsidiary character. In "Cat Among the Pigeons", Poirot's entrance is so late as to be almost an afterthought. Whether this was a reflection of his age or of Christie's distaste for him, is impossible to assess. "Crooked House" (1949) and "Ordeal by Innocence" (1957), which could easily have been Poirot novels, represent a logical endpoint of the general diminution of his presence in such works.
text_B: Is the exact reason Christie did this known?
NO.