God gave dominion of the earth to Adam. Genesis 1:26 says, "Then God said, "Let Us make man in Our image, according to Our likeness; let them have dominion over the fish of the sea, over the birds of the air, and over the cattle, over all the earth and over every creeping thing that creeps on the earth."
In that perfect world, the devil entered the Garden of Eden through the nature of the serpent (Gen 3:1). It was Adam's responsibility to do the right thing, not God. Adam had legal ownership of this earth including everything on it. Adam chose not to exercise that legal right, and in the process, lost the rights of ownership (including man himself), giving it to the devil (Matt 4:8-10).
From that very moment of Man's fall, God provided a plan of redemption (Gen 3:15). This redemption came to fruition through Jesus Christ. Jesus demonstrated the kind of life Adam would've had had he not sinned and showed what human beings can be like. Jesus lived a perfect life and brought forth salvation and in the process showed the principalities and powers that love, not domination power, triumphs!
So, it wasn't as simple as to just toss the devil and his cohorts into the lake of fire. What was lost needed to be redeemed back--legally. And God did just that, through His Son, for the love of the world (John 3:16).
Hope this helps somewhat, God bless!