Why doesn't or hasn't God just imprisoned Satan so no one has to go to hell?
Satan will face his one judgment at the time appointed by God. After the rapture of the Church - when all who have accepted Christ as Savior with all that entails are taken to heaven, Satan will have a field day on earth. During this time God will be pouring out judgment on earth. When the roughly seven year period is complete, Satan will be bound for a 1,000 year period. After this millennial period, he will be loosed again, and will go forth and deceive everyone to try to finish what he started. At this point, he will be ultimately defeated and his doom is forecast in Revelation 20:1-10.
When Satan lost control of the planet originally in his war against God, Adam and Eve were created and given dominion. We don't know how long they lived there before they disobeyed God, but eventually they did. From that very first time, God shed blood to cover their bodies. It was up to Adam and Eve to propagate the knowledge of God and His standards to their children and it has been the duty of every parent to do that since. Clearly, they failed since Cain slew his brother Abel. Likewise, the requirements of God for salvation have been either successfully or unsuccessfully passed between parent and child ever since. So there has always been a way that God's creation could escape having to go to hell. Today we have at our disposal vast quantities of Bibles, religious texts, and internet sites that provide the Gospel Message to help people be saved and to give them resources as to how to learn about God so they won't go to hell. And even without these things at your disposal, the Holy Spirit works in every heart as the "conscience" telling you what is the right and wrong thing to do. Eventually, you can kill this voice if you try hard enough, but from the very beginning He is present trying to lead each individual of His creation to salvation. No one has to go to hell. If they do, it is their own choice.
The thing is these two groups are separate. Satan and the other angels that rebelled against God have their own future judgment, and we have ours. Just locking Satan up wouldn't eliminate the problem of sin in our life. We like to use the expression "the devil made me do it" but that is never the case. We are led away by our own desires and fleshly lusts to do wrong, and it is our own natures that we need to die to daily. Locking Satan up wouldn't fix that problem at all. It may reduce a certain amount of the resistance that the spreading of the word faces, but it won't fix the sin problem. Even after Satan is locked up for 1,000 years, he is still able to convince the people of earth to join in his rebellion against God. That is proof positive that just locking him up wouldn't solve anything. After 1,000 years of Christ's rule and all the good that will come of it, the peace, the plenty, people will still remember and long for their sin enough to join in a rebellion against God. While that doesn't seem possible, it is what the Bible declares will happen.
Each person must decide for themselves what their eternal destiny is. I will grant that Satan does spread a lot of confusion and has brought down many in the various churches and spread many false religions, but much of his success is because the Christians of today aren't doing their job in spreading the word about what God is doing for them. Regardless, it is clear that just locking him and his angels up won't solve the problem.