Why is Satan bound during the millennium?
Satan is bound during the millennium so the new generations born in that thousand-year reign have the same opportunity Adam and Eve had in the Garden. He is locked up to make that time special again, as people live under Christ's perfect rule.
Les explains that after the age of grace ends and God pours out His wrath in the final seven years, the millennial reign begins—a thousand years still in time, with men working, marrying, and having children under the perfect absolute rule of Christ. In order for this to be a special time again, Satan is locked up to give all these new generations the same opportunity Adam and Eve had. In the Garden, Adam and Eve had everything perfect, lacked nothing, and could not have wished for better spouses. Satan's binding restores that kind of opportunity for those born during the millennium.