Because God is all-powerful, he certainly could have prevented the fall of the angels and the subsequent fall of Adam and Eve. God does not explain why he did not prevent those falls—nor is he obligated to do so (Romans 11:33-36).
After the devil led Adam and Eve into sin, God promised a Savior (Genesis 3:15). That Savior would crush Satan one day. By his holy life, sacrificial death and glorious resurrection, Jesus did crush Satan. The devil’s final defeat will take place on the Last Day, when he will be confined to hell forever (Revelation 20:10).
Until the Last Day, Satan retains power and influence as allowed by God (Job 1:12; 2:6). Because of his power and influence, the devil has the title “the prince of this world.” (John 12:31). As mentioned previously, the day will come when that “prince” will be crushed completely and be unable to bother God’s people anymore.
Until that time, God’s people recognize the power and evil intent of Satan (1 Peter 5:8), and they use the spiritual weapons God has given them to be successful in warding off the old evil foe (Ephesians 6:10-18; James 4:7).