Why do author use the same detective in their stories?

Nothing succeeds like success. If your reading public read something of yours and liked it and want more then you write the same again and hope that they appreciate it just as much.

Agatha Christie invented Hercules Poirot and he was the detective that her readers really liked. Unfortunately she didn't. Was saddened that he was the one they latched unto but kept writing him to please her public.

Arther Conan Doyle tried to kill Sherlock Homes off when he got tired of him but had to bring him back to life because of his readers.