Pity the poor apostles; they have met this wonderful man who has taught them and shown them so much that they can't take it in. He has died and has risen. Now he teaches them so much after the resurrection that they have a head full of knowledge about Jesus and God. Soon Jesus is to go, tomorrow he shall ascend to the Father. They are still unsure. It is not until the Spirit comes and enlightens their minds that they understand.
Now they have knowledge, then they shall have experience; they shall know in their very bones that Jesus is Lord and this shall utterly change their lives. That's what baptism in the Spirit does to us. But first Jesus must leave and the apostles must wait!