Due to the nature of how translation works, interpretation will always be inherent to it, especially when we can't ask the original author for clarification. As for the church wanting the faithful to know more about demons, you could argue that is why they borrowed so much demonology from other sources. As for why it wasn't included in the Bible itself, any answer will have to be heavily seasoned with speculation. It is important to remember, however, that the New Testament was not intended as a "how to get to heaven" or "how to avoid hell" guide. It was intended as telling the story of why you get to go to heaven in the first place (the Gospels) and letters discussing how to live faithfully. Plus Revelation.
Off the top of my head, the Bible really says surprisingly little about demons, and various translations will say even less (depending on if the translator thinks an evil spirit=demon or not). Generally, the possessed act either mentally or physically ill, or they get scared of Jesus. Beelzebub is the prince of demons, but that's about all we know from accepted scripture. Most "Christian" demonology stems from writings that didn't make the final cut for the Bible, mythology borrowed from other cultures/religion, bored/crazy monks in a position to write things down, and particularly influential works that never claimed to be anything but fiction (namely Dante's Inferno).