This show is a very good show, it had a huge record, though the executives at Nickelodeon tried to cancel the show 5 times, it's good to watch. I think season 9 is a terrible season but has 5 good episodes. Season 10 on the other hand is god awful to watch. Here are some reasons why the show failed.
1. Franchise Fatigue
The show was running out of ideas so they tried to add new characters to get their old audience to come back, but this resulted in a massive failure, ending up in the show's cancelling of June 2017.
2. The Animation
The animation was reverted from traditional to flash, and it is AWFUL. The worst part is that Poof never showed up in season 10.
I give this show a 5/10 rating for great graphics, unique story telling (Seasons 1-8), and the downside is that the show went on for far too long, and that's why it got cancelled. It's a good show to look back at, and I hope you guys say the same.