Creative fulfilment: Rowling has stated that she felt creatively satisfied with the story she had told and did not want to continue writing about the characters beyond what she believed was necessary to complete their journey.
Character development: Rowling wanted to ensure that the characters had a complete and satisfying arc, and she felt that the seventh book brought their stories to a natural conclusion.
Maintaining the quality: Rowling set a high standard for her writing and wanted to maintain the quality of the series. She felt that continuing the story beyond seven books might compromise her ability to deliver a satisfying conclusion.
Work-life balance: Rowling wanted to have more time for her personal life and other projects, including writing outside the Harry Potter universe.
It's worth noting that Rowling has been clear that the story of the seven Harry Potter books is complete, and she has no plans to write additional books in the series. However, she has expanded the Harry Potter universe through other works such as the prequel series "Fantastic Beasts and Where to Find Them", short stories, and companion books that provide additional context and information about the world of Harry Potter.