This might've been a time ago, but the only reason Edo Itachi is still alive is because "Two genjutsu can't overlap each other." He was under the effects of Kabuto's will and Kotoamatsukami. In which case, both cancelled each other out and he was henceforth free, so that Kishi could keep up his streak of no one being able to defeat him a.k.a. Naruto breaking his foot off. So, why did it happen to Kabuto? He was under the effects of Izanami, which Itachi says is a genjutsu, yet, straight from Narutopedia,
"Outside Kabuto's mind, after his glasses were removed, Itachi proceeds to cast another genjutsu to force Kabuto to reveal the hand seals needed to cancel Impure World Reincarnation and then had him execute the release."
They should have both either canceled each other out, and Kabuto was from there free just like Itachi, or Itachi would have to cancel Izanami, and due to Kabuto's threat perception, be able to counter Itachi's next genjutsu.
Make no mistake, Kishi breaks the rules so Itachi doesn't get defeated by ANYONE!