The Bible presents God's kingdom as gradually growing, but not instantly defeating the Kingdome of Satan. The Devil took kingship of the world because he tempted the first humans to sin. When they fell they fell into sin, death, hell and under the bondage of the malicious lordship of Satan. (Genesis 3:1)
Gradually over history God established a new kingdom. In Abraham he called out a people to live under his rule. (Genesis 12) Then in Christ he made a new heaven and earth creating a new kingdom (Isaiah 65). This is why Jesus always said the 'Kingdom of God is like...' (Math 13) and John the Baptists said the kingdom of God was at hand. (Mathew 3:2)
Now when Jesus did arrive the Devil was enlivened in terms of Biblical descriptions of his presence in the world (Math 4:1) and all his minions. Before Christ there is very little spoken about the devils or the Devil. (Math 8:16)
Now after Jesus rose, he promised to return in judgment. (2 Timothy 4:1) This is the final building of his kingdom when sin, death, hell and Devil will be bound forever. (Rev 21:4)
So you see the question you asked is not why still the Devil, but why sin, why disease, why sorrow, why not heaven now?
The answer is God does all things according to his own eternal plan and this apparently includes a gradual salvation of humanity in Christ. (Eph 1:3)