Because a series becomes stale if you keep repeating it. Many series have fallen victims to going on too long. Its better creating a new franchise with new adventures than continuing to repeat the same thing over and over again. People will find those games boring and stale. Instead they can improve on the current games, like a remaster for Witcher 1 and similiar for new systems. I don't believe the story of the witchers will end just yet however, it takes time to write new material and new adventures. Geralt may finally take his break, but new witchers may yet be created.
I really find the witcher story interesting, would be cool to create a witcher of your own from a school, starting young, growing up, doing trials and going monster hunting and helping people. Similiar to what they are doing now with Cyberpunk. Maybe something similiar to Fable?