Because if he was in Super Saiyan god's Destruction form, then even if the surrounding space-time could be destroyed by the energy of the Destruction Rule, Robet would have to pay an even greater cost in energy and rule power.
Just as Lucifer had thought, even though Robet was rather special, his energy was not truly infinite in the absolute sense. It was just that so-called energy consumption did not affect Robet's strength.
And very clearly, Ultra Instinct consumed far less of Robet's energy than Super Saiyan god did when using the energy of the Destruction Rule.
So although the Ki Blasts in his Ultra Instinct state were much weaker than the destructive energy of Super Saiyan god, Robet truly had no other choice.
This could also be considered one of Ultra Instinct's special traits. At least for the current Robet, aside from ki, this pure form of energy, no other attribute-based ability could be used while in Ultra Instinct.
Yes, with Robet's current strength, he could already use destructive energy in his normal state, yet he still had no way to use this move while in Ultra Instinct.
This was probably a matter of attribute incompatibility, just like how Sentry, in his normal state before, had been unable to use the power of Void, and in his Void state, had likewise been unable to use his original power.
"Damn it! What is going on?" Lucifer could not help frowning at the sight. Within the Fallen Angel Universe, he was nearly omnipotent, yet right now he had no way to deal with Robet at all.
At present, all Lucifer could do was continuously alter the space-time of the current universe. This was not to affect Robet and thereby restrict him, but to make his attacks miss.
As for transferring Robet