Why doesn't God destroy the devil forever?

There are somethings to keep in mind before we approach this question....
1. God is not powerless against the devil and can destroy the devil at this moment if he wants to....
2. God is all wise and in his wisdom if he chooses to do something, it means that he has some pretty good reasons to do so.... The reason I state this is because, from our human perspective (even I do wonder the same at times, so I completely understand you when you ask this question), It might seem like the best and easiest thing to do... no devil and no temptations, no sins , no evil...what a happy world right? but wait...that's for us... if God thought it was the perfect thing to do, he'd have done it already... But God doesn't.. the temptations and struggles also is something that God is able to use for good... in fact we tend to depend more on God when we are to overcome something or some struggle.. also ..when there is a temptation and we fight it and make a choice of running to God, that we are able to obey God... When there are other things that may take away our love for God, but we reject it, and continue to obey God, God is glorified in a much greater manner....
3. Look at the gospel story itself.... God used the fall of man,and the work of the devil in the lives of many people by stepping down and dying for us...that's a big demonstration of God's love for us... these are some visible reasons why God maybe choosing to delay the ultimate end of devil...
4. And most importantly, remember... God will ultimately defeat the devil and he has promised us a perfect life which is eternal... and He will do it, when He thinks is best...
Ending with a helpful quote from pastor John piper..
"The glory of God and Christ shines more brightly when we are seen to be supremely satisfied in Christ in spite of Satan's torments, rather than if we had his torments removed and liked Jesus because of it.
It's when you love Jesus in spite of Satan's torments and through them that his glory shines most brightly, rather than when we have life made easier for us by Satan's removal and we like Jesus because of it."