Because sin was something completely new in God’s creation, and its inhabitants did not understand it. Likely, even Lucifer did not fully comprehend it at first. As a brilliant, respected leader, Lucifer could present himself as one deeply concerned for heaven and the angels.
One third of the angels joined Lucifer. Lucifer’s arguments convinced many angels, and a third joined his rebellion (Revelation 12:3,4). If God had destroyed Lucifer immediately, some angels who did not fully understand God’s character might have begun to obey out of fear rather than love.
God desires only loving, voluntary service. The only service God desires is cheerful, voluntary service prompted by genuine love. Obedience motivated by fear is ineffective and eventually leads to sin.
God is giving Satan time to demonstrate his principles. Satan claims he has a better plan for the universe. God is giving him time to demonstrate it. The Lord will abolish sin only after every soul in the universe is convinced that Satan’s government is unfair, hateful, ruthless, lying, and destructive.
The universe is watching this world. The Bible says we have been made a spectacle to the world, to angels, and to men (1 Corinthians 4:9). The whole universe is watching as each of us plays a part in the controversy between Christ and Satan. When it ends, every soul will understand the principles of both kingdoms and will have chosen either Christ or Satan.