Perhaps it is because that previous to Christ it was NOT possible to 'cast' them out. Therefore, the less that 'these' were mentioned, the less people would understand them. Kind of like the 'forbidden fruit'. I took satan and his influence to deceive mankind into sin. God simply said 'don't', yet satan was the tempter through offered knowledge.
In other words, perhaps God didn't offer much information on demons because He knew that for us to remain ignorant would keep many from following demonology on purpose. Once satan had revealed much of this knowledge, there was no longer a purpose to conceal it but rather a need to expose it once there was a way to fight them.
There is much indication that demonology goes way back. The cabal was created by pagan civilizations way 'before' our Bible or even the Torah. This 'witchcraft' is what Moses warned his people about. You can be assured that there were certainly demons BC. There is MUCH indication that at times there were whole cities peopled by those that had 'given in' to demons. It becomes quite indicative that when a 'whole city' takes part in the 'same evil' that there is 'something common' to them all. What could possibly influence a 'whole people' to take part in the 'same evil? Demons my friend.
I believe that 'His' people, the Hebrews, had been kept ignorant of this witchcraft until they had become enslaved by the nations that practiced it. But, once they had become an integral part of their 'slave masters' culture and religion, they became aware of it and many began to practice it. Hence, the reason that Moses mentions 'familiar spirits', 'witches', 'sorcerers', etc......
Let us compare the psychology of 'drug' use to demonology for a moment. if parents could keep their child from even knowing that 'drugs' exist, look how much easier it would be to prevent their children from ever taking part in the ritual.
Now, let us use this same situation when attempting to answer your question. The less someone knows about something, the less likely they are to 'find it' or 'participate' in it. Maybe this is why 'God's chosen people' were seemingly ignorant of 'demons' throughout the OT.
And this would go a long way towards a better understanding of 'why' God wanted His people to remain 'separate' from others. The more and longer that they could remain 'separate' the less they would be exposed to the influences of others that had learned to incorporate 'satan' and his 'minions' into their culture and religion.
I don't offer any of this as fact. I have studied a bit on the subject however and this is what I believe may have been at least a part of the answer.
