Julianna was only 10 years old when she started playing Stephanie, and as she grew older, she desired to explore other roles and projects. She also wanted to pursue schooling and her personal life, which were difficult to balance with her busy acting schedule.
While the exact reason for her departure was never officially confirmed, it's generally understood that it was a mutual decision between Julianna and the show's producers.
After Julianna left, the role of Stephanie was taken over by Chloe Lang, who played the character for the remaining three seasons of the show.