Why doesn't God throw Satan into the lake of fire during the millennium?
Satan is confined to the bottomless pit rather than the Lake of Fire because the Lake of Fire is a place of eternity, and Satan is not yet ready for eternity. He must be released after the thousand years for one final opportunity to deceive the nations, making the millennium a seventh period of testing.
Les explains that God confines Satan in the bottomless pit, a unique place known only to God, for the thousand-year period. The reason Satan cannot be sent to the Lake of Fire is that the Lake of Fire is a place of eternity, and Satan is not ready for eternity yet. He has one more opportunity at the end of the thousand years: he will be released and given full freedom to deceive the nations once again. Therefore, rather than putting him into his place of eternal doom, God confines him until the thousand years are fulfilled. After that, Satan must be loosed for a little season to test the new generations in the kingdom economy, making the millennium another period of testing—the seventh, since God always does things in seven.