Several reasons, here's a comprehensive list:
- Downgraded animation.
- The plots are more generic than before.
- The future setting is an obvious attempt generate more interest, rather than a genuine change for the better.
- It barely connects to the previous seasons.
- The story is no longer as tightly knit.
- It required an entire new cast of characters, most of whom sucked.
- The toy advertisement increased even further.
- While there is an excuse, having the Turtles constantly run around in daylight like it's the Fred Wolf show just feels wrong.
- It didn't really conclude. It just left hanging plot threads.
It's not the worst thing ever, you could even argue BttS is worse, but it's not good. If there's a silver lining to this season, it's that Dunn came from it, who was actually kind of a cool character when written by Tom Waltz.