One possible reason why the Daevas became demons in Iran is this: At about the same time as the Indians were writing the last part of the RigVeda, a very thoughtful man named Zoroaster appeared in Iran. He thought and thought a lot about why there was goodness and badness in the world. Finally, he told everyone what he had been thinking about. And one of those things was that Daevas were not to be trusted because they could not tell the difference between truth and lies. After some time Zoroaster became very popular for his ideas. And so, with time, the Daevas became demons in Iran. 