Because Revelation was written by a Greek, not by a Jew. "Satan" isn't a person in the OT, it's a title, humans and angels are both referred to as "satan" in the OT, while "lucifer" is just a translation of a Hebrew name for Venus (the planet), which is called the Morning Star because it often appears just before dawn, ie, it is the "light-bringer".
The idea that Satan is a singular being at odds with god is found nowhere in the OT, and is anathema to Jewish traditions. It's an echo of Persian dualism that found its way into Greek mysticism. The conflation of Venus with Satan is more mysterious, and seems to have developed within the early Church.