It is not at all easy to understand.
Jesus sat on the throne of David - Gabriel told Mary:
Luke 1:32 He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David:
This was God's promise to David, that The Messiah would sit on the throne of David. David was King of Judea first and then King of Israel.
Jesus ascended to the throne when he ascended to heaven:
Hebrews 1:
8 But unto the Son he saith, Thy throne, O God, is for ever and ever: a sceptre of righteousness is the sceptre of thy kingdom.
So when he ascended, he was crowned in heaven by God. This was the Kingdom of Heaven. (To be a Kingdom you require a king).
However God had vowed to take the Kingdom of God from Israel
Matthew 21:43 Therefore say I unto you, The kingdom of God shall be taken from you, and given to a nation bringing forth the fruits thereof.
So it was impossible for Israel to have the Kingdom of God. That is why Matthew, addressed to the Jews, refers to the Kingdom of Heaven, rather than the Kingdom of God.
When Israel was destroyed in AD 73 by the Romans, and ceased to exist, then the Kingdom of Heaven could now be called the Kingdom Of God, and was given to all nations.
That is why in the Temple Prophecy (Olivet Discourse) Jesus said that once Jerusalem and Israel had been destroyed, then the Kingdom of God could come:
Luke 21: 31 So likewise ye, when ye see these things come to pass, know ye that the kingdom of God is nigh at hand.