Why did the Holy Spirit come down after Jesus ascended to glory?
The Holy Spirit came down after Jesus ascended to glory as a transition of power, to continue dealing especially with the nation of Israel. This followed God exalting Jesus to be a Prince and a Savior, to give repentance to Israel.
Les explains this as a transition of power: the Lord Jesus goes up to glory, and the Holy Spirit comes down to continue dealing especially with the nation of Israel. He points to chapter 5, verse 31, where it says the God of our fathers raised up Jesus, whom they slew and hanged on a tree. Him God exalted with His right hand to be a Prince and a Savior, to give repentance to Israel. So the Spirit's descent follows Christ's exaltation and continues God's dealings with Israel.