At the risk of sounding flippant, why not? For what reason or reasons should God have not created the devil even if he was going to fall? Just because God knows what will happen doesn't mean that the person (or angel) isn't free to make choices. Satan freely chose to rebel against God. God knew this would happen. Nevertheless, let me offer some possible reasons why God would create Satan even though He knew he would fall and rebel.
- It was necessary to have the fall so that God could then have a reason to die for our sins thereby demonstrating that God can and does provide the greatest act of love which is to lay down one's life for his friend (John 15:13).
- The fall of Satan provides yet another method for God to be glorified in that God can use sin to prove that sin is "bad" and that God's Word about righteousness is true.
- If God is to have creatures with free will, then the risk of rebellion is part of that freedom. Satan had that freedom and used it to rebel.
- If God had not created Satan and instead another angel fell, then we'd be asking why God made that angel knowing he would fall.
- God has reasons about which we know nothing.
I know my children will act badly at times, but knowing that does not mean that I shouldn't have kids. Part of the risk of freedom is that rebellion will be a reality.