Part of DBZ was the transition from Anime to Manga.
First he did Dragonball, and that did well, so I think he decided to do DBZ to go off that.
When DBZ Anime finished Frieza, there was this whole, NUUUUUUUUUUU, cause it was so popular, so they keep going, whereas the Manga only went to that point.
Then after Cell I think he wanted to wrap it up again, but was probably told to keep going, and I personal think the Buu Sagas weren't as good as the rest, even Cell to a degree. Basically the same each Saga, big badguy comes, have to transform to a new level and pwn all.
The GT... that was only part the original guy I think.