My guess as to why the good news of the kingdom is muted, is not because pastors don't understand it. Look at how many pastors have ridiculously tried to decipher the prophecies in Revelation, each with a different set of predictions with how the prophecies would unfold.
My guess as to why many pastors remain silent on the good news of the kingdom, is that it doesn't fit into the tiny neat little formula that the gospel is the Death, Burial, and Resurrection of Jesus Christ, and nothing else.
The script goes as follows:
"The gospel is simple, God loves you and wants a relationship with you. You're a sinner and Jesus paid the price for you on the cross. Just accept His gift."
If pastors/preachers/ministers were to instead preach as follows
Acts 8:12 But when they believed Philip as he proclaimed the good news of the kingdom of God and the name of Jesus Christ, they were baptized, both men and women.
Acts 28:23, 31 They arranged to meet Paul on a certain day, and came in even larger numbers to the place where he was staying. He witnessed to them from morning till evening, explaining about the kingdom of God, and from the Law of Moses and from the Prophets he tried to persuade them about Jesus. [31] He proclaimed the kingdom of God and taught about the Lord Jesus Christ---with all boldness and without hindrance!
Where they taught about both Jesus and the kingdom "Together", then it would interfere with that tidy little formula. The gospel would no longer be as simple as they want it to be, and are used to.
The kingdom is an Encumbrance and must be excluded from regular preaching/teaching of the gospel, except on special occasions.