Sasuke is fighting elsewhere, how could he possibly protect her… especially after his susanoo just got destroyed by one-handed Kaguya loooool.
However, in this case; Sasuke is acting as a diversion so that Naruto can attack since most of his jutsu from the mark on his hand physically works on her (yang). Sasuke's part is more of sealing her and shielding from her spiritual techniques (Yin).
Anyway As he was about to use his eye power, he noticed Kakashi's Susanoo. Kakashi got his perfect susanoo by copying the Sasunoo that Sasuke had.
That is; since he is able to copy any jutsu (which is one of the powers of Obito's eyes), plus the double MS grants him Susanoo anyway… but instead of doing a normal susanoo he copied Sasuke's (if you look at the manga page, that's the first thing he saw).