Mainly because the writing team and production values changed. The saiyans were treated far differently as Goten got no character development, Trunks lost his edge, and Gohan suddenly reverted back to a SSJ. Krillin aged incredibly despite being younger than Yamcha and Tien. Piccolo really had no role for the first time. Also wasted Uub
It could have been called the Vegeta and Goku show.
The one bright side though was Pan to me and the fact that the fighting wasn't bad against Super 17, Baby, and Omega.
I think they should have honestly sent Bulla off with Goku and Pan into space in all honesty if they weren't going to put more time into Trunk's character