He doesn’t want to destroy ISIS.
I absolutely believe he finds ISIS a distraction. Even FDR had enough sense to know that when the Japanese bombed Pearl Harbor that it became a top priority. Obama, no doubt sees their threat as a perception problem. He would no doubt rather Americans ignore it. The outrage over the death of Foley and genocide of Christians in Iraq caught him by surprise.
I wouldn’t be surprised if Obama consulted with ISIS before his speech.