Because filler, DBZ filler added to the show, intensifying fight scenes and epic showdowns, Naruto's dragged on forever even when the manga was finished and there was no reason for filler other than greed, because they know we'll keep watching and buying, even if the quality drops. DBZ ended with lots more potential to grow, DBS and GT proved that, Naruto did not. Sure we have all the kids but what we loved about Naruto were the characters, get rid of all the characters that we love and why even bother? Everything has a time to end, DBZ has not reached it, it was hardy 300 episodes, with filler, Naruto far exceeds that mostly because of filler, and reached its conclusion. It's final villain felt very final. It was clear that this was the end. Buu... just felt like another DBZ villain, he could be replaced or beaten, by a stronger villain. No one said after Buu that he couldn't be surpassed, heck we even got Uub. In summation, these are two different anime/manga that had different impacts, times and feels. Comparing them ia pointless. Hope that answered your question.