Why did God cast the fallen angels down to earth to tempt mankind?

Your question assumes this happened long ago, before God created the world, in a great battle among the angels; a lot of people believe that, and it is false. There was a conspiracy by Lucifer, and Apocalypse 12 tells us in symbols that he took a third of the angels with him, which is a lot. He must have convinced them he could win, trapping God so that God would have to give the world up to him. What he missed was that God would become a man and deal with the problem; that was the biggest shock of his life. In making Adam and Eve sin, Lucifer won a partial victory, so that if nothing were done to redeem them, Satan would rule them, which is why Satan was not immediately destroyed. That was taken care of at the cross. Apocalypse 12 says there was war in heaven, Michael and his angels fought the dragon, and the dragon was hurled down to the earth; this happens at the time of the cross, when Jesus said in Luke 10, I saw Satan fall like lightning from heaven. Before the cross, as in the Book of Job, Satan still had access to heaven; after the cross he was cast down. That is why Paul calls them principalities and powers in the air.