Because man needed a choice. God told Adam and Eve not to eat the fruit of the tree and they would not have. However, the devil only had to suggest to Eve, who then convinced Adam that it was acceptable to eat the fruit, this gave them a choice.
To choose between their love and obedience for God, as the angels also did in heaven. Hence those who sinned in heaven were cast out, as was Adam and Eve cast from the Garden of Eden.
This is why i believe God did not destroy the devil. The devil was the tool used to initialise the first choice of freewill in man.
Man may not have rebelled if the devil did not exist, but then, freewill in man would not exist either.