First of all, Kabuto was bating the shit out of Sasuke and Itachi until Kishi made an asspull and made Izanami.
Second of all, It was Nagato vs. KCM Naruto, Bee, and Itachi...
...and Nagato was shitting on who he could fight.
Nagato couldn't attack Itachi because Edos are programmed not to...