I'm going out on a limb to speculate. After the creation was finished, Lucifer decided that he wanted (for whatever reason) to ascend above God. God told him, OK, let's try that, and I'll cut you 6,000 years to prove you can do better. After that, if you fail, I'll reassert Myself. I think God did that knowing that the result would vindicate Him before the witness of the whole created universe.
We have only glimpses of all this. As Satan, Lucifer may have overreached himself, and there was war in heaven, but we know little of it. We lack knowledge of territorial limits, rules of engagement, how long a leash God put on Satan, etc, etc, etc.
What is important to keep in mind is this: Satan would like us to think he is the god of this world, but he is a usurper, a pretender to the throne which rightly belongs to Jesus.