There are several reasons for this:
* Show's natural progression: The show ran for a good amount of time, and its creators likely felt they had told most of the stories they wanted to with Jimmy and his friends.
* Success of the films: The franchise had two successful films - "Jimmy Neutron: Boy Genius" (2001) and "Jimmy Neutron: Win, Lose or Draw" (2002). These likely brought a sense of closure to the series.
* Change in animation trends: Around this time, Nickelodeon was shifting its focus towards shows like "SpongeBob SquarePants," which were more successful with younger audiences.
* Overall, the show was successful: It won numerous awards and was beloved by audiences. The creators may have simply decided to end the show on a high note rather than risk it becoming stale.
So, it wasn't a cancellation due to poor ratings or a lack of interest. The show was simply allowed to end its run naturally, leaving a legacy of fun and inventive stories about a boy genius and his quirky friends.