Perhaps before this, they hadn't considered using the Authority of Creation to deal with the Lord of Mist.
It wasn't until Earth Consciousness unexpectedly fished out a piece of the Lord of Mist's flesh imbued with the God of Order's will from an unknown fishing spot in Subspace, that Xue Dili and the God of Order hit it off and decided to absorb this flesh of the Lord of Mist and take control of the Authority of Creation.
This was also due to Xue Dili's consistently unabashed adoption of other powers. He never cared about the source of his power, taking whatever he could use regardless of whether it was dark or light, and no matter the side effects, as long as it was useful.
Moreover, another point that other Apostles couldn't match was that the authority within Xue Dili was exceedingly complex, which was both his bane and his ace enabling him to reach this stage.
