KyoryuGold also was supposed to be stronger than everyone else too. KyoryuGold by himself keeping up with a powered up Dogold would have made perfect sense, no need to drag Daigo there. And it's not just that battle. When the monster of the week appeared, the role of the other Kyoryuger became just to give batteries to Daigo.
There's a difference between Sentai's standard Red focus and what's going on here. Gokaiger had Red focus. GokaiRed beat Basco after his entire team was defeated by him, but he didn't go around taking over battles in episodes that weren't even focused on him.
KyoryuGold was pre-Endolf Dogold's equal/rival. Even before this episode, he was almost always sidelined out of team fights to duke it out with Dogold.
It would make absolutely no sense that he would still be a match for EnDogold. At least, if we assume that the new Dogold is much stronger, which is a fair assumption to make.
EDIT: On the second point, Kyoryured was already in Carnival. And Toei needed to shill out more toys. Logical next step?