John Ortved nails it regarding what went wrong with the Simpsons:
As the show sort of moved away from its roots, starting around the sixth season, and the show kind of got a little zanier, the show became sort of unmoored from those emotional character-driven plots that initiated the series. You really start to get 21 minutes of throwaway jokes and then one minute of emotional reconciliation thrown in at the end.
The show went from clever and well-written, to a prototype for Family Guy.