There is a range of answers to that question. One answer is that Christ is delaying His coming, so to speak, in order to give people an opportunity to repent. The second is that Christ came into the world to overcome sin, to overcome death, and to overcome Satan, but He also came into the world so that sin, death, and Satan would be overcome in our lives. That overcoming is a progressive reality for us, not an instantaneous reality.
So, for example, think about the notion that Christ defeated sin. Paul teaches in Romans 6 that when we’re united to Christ, we’ve died to the dominion of sin, but sin hasn’t yet died in us. One reason sin hasn’t yet died in us is that growth in grace is a necessary part of the Christian life. We can also say Christ has defeated Satan, and yet Satan still troubles us. Why does God allow this? He allows this because He wants to see us progress in grace because He is our Father.
So, you might think about it this way: the father who removes all challenges from his children, or to put it another way, the father who has a fabulous amount of money and says, “This is your inheritance, and I’m going to give it to you now,” would not be a wise father. He wants to see his children grow into that, and that’s what the heavenly Father wants to do.
Sometimes He takes away some of our friends much earlier than we would want, and we sometimes wonder why. Perhaps it is because He simply wants them in heaven, or perhaps it is because He has some mysterious purpose, but characteristically, He wants to see us grow from being infants to being spiritual adults, and we can’t do that without learning how to overcome the enemies of sin, death, and Satan.
Think about it this way: if we were immediately taken to heaven and there was no challenge of facing death and releasing everything we have, everything we are, everything we love, and the people we love into His hands for His care, there are many things about learning to trust Him that we would never discover. So, there are flashes of illumination both in Scripture and our experience about why He has sovereignly patterned the Christian life this way.