“Why” was the easiest question to answer. From the beginning of time, God’s desire has been for his dwelling place to be with man. We see this as he walks with Adam and Eve in the garden; in His desires to come near to Israel, but for fear, they send Moses as a go-between; in His instructions for the ark and the temple; and in the revelation of his final solution to the world’s sin which he spoke through the prophets.
John 3:16 answers the question of why the Kingdom of heaven as come to earth. It says:
“For God so loved the world, that he gave his only begotten Son that whosoever believes in him shall not perish but have everlasting life.”
Why has the Kingdom of Heaven come to earth? Because God so loved the world. He sent his only Son, Jesus, to earth. His birth, life, ministry, death, and resurrection inaugurated a new era in history which accomplishes all that God has desired from the beginning . . . His Kingdom coming to earth; Himself dwelling with man.