Because, he couldn't. He'd used it prior on another mission and it was on ten year cool-down. It wasn't ready to use until Edo Itachi came back. Their plan of KA'ing the coup was basically wrecked by Danzo stealing the active one.
better question is why didn't he go to the hokage and tell him "hey your underling danzo kinda just STOLE MY RIGHT DAMN EYE" and had him removed from office
better question is why didn't he go to the hokage and tell him "hey your underling danzo kinda just STOLE MY RIGHT DAMN EYE" and had him removed from office