Beyond that, why did Jesus ascend? His goal was not just to build something for us on earth. His goal was to bring us to heaven. He can't stay here for the same reason we can't stay here. We are made to be with God. We forget that. We get so caught up in out day to day struggles we lose track of what this life is all about. The most important outcome of this life is determining whether we go to heaven or hell. Everything else pales by comparison. Jesus is in heaven. He is fully human. That means if He can go we can go. There is nothing we should want more.
The other reason Jesus ascended is so that the Holy Spirit could come. The Kingdom of God is not simply going to be brought about by Jesus. He wants to include us in that mission. That happens through the Holy Spirit. That is where we get power. That is how we become witnesses. It is how the grace of God can spread and renew the face of the earth. So we get something worthwhile to spend out lives on.
Think about the gospel:
All power in heaven and on earth has been given to me. Go, therefore, and make disciples of all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit, teaching them to observe all that I have commanded you. And behold, I am with you always, until the end of the age.This is called The Great Commission. The striking thing about it is the absolutes. Jesus claims all power. He sends them to all nations. They are to baptize into the trinity which is all of God. The disciples are to observe all commands. Jesus will be with them all the time. Nothing is partial. It is not the best they can do. They must change the entire world. They must change it completely.
Think about it. They are just a few. Acts says about 120 people. They are not rich. They are not especially talented. They don't hold high office. They have just recently learned about God the Father. Jesus they know but the whole death and resurrection thing has blown their mind. The Holy Spirit? They know almost nothing about Him. Yet they are going to teach the world this gospel?
This is the task we have to. Something we are totally incompetent to do. Yet Jesus says He will be with us. Never mind how you are going to change the world. Just go. Go and I will be with you. What more do you need to know?