Because Revelation is a book of prophecy, not history. Hence, as of John’s day, Satan had not yet been cast down to earth.
(‏ كشف ١ :‏ ١‏ )‏ اذًا ،‏ في زمن يوحنا لم يكن الشيطان قد طُرِح بعد الى الارض .‏