Agatha Christie was a really poor writer.
I agree with you, but that was part of the appeal - Dorothy L. Sayers was the far more accomplished crafter of mystery without needing to revert to nonsense like people trapped on an island long enough to be murdered but not rescued. Her Five Red Herrings is the pinnacle of the genre, but people don't want hard to solve, difficult to follow stories. You solve the mystery, you buy the next book because it makes you feel good about yourself.