It did better than sell well , it broke all sales records!
SPARE was a huge success because it revealed information about the RF, in particular Kate, Wills and Camilla. Most people, if they're honest, were reading it for dog bowl not psychotherapy.
Harry just doesn't have access to those people any more. Perhaps there's still a bit of material he can use from the period leading up to the Queen's funeral, but it must be getting a bit sparse from around the time of PP's funeral onwards as his family stopped wanting to talk to him.
So perhaps one more book with interesting revelations in it. Then what?