Why did they call Jesus, “Son of David”?

God spoke to King David through the prophet Nathan:

"And when thy days be fulfilled, and thou shalt sleep with thy fathers, I will set up thy seed after thee, which shall proceed out of thy bowels, and I will establish his kingdom. He shall build an house for my name, and I will stablish the throne of his kingdom for ever." - 2 Samuel 7:12-13

God promised David that Messiah – God manifest in flesh – would come through David’s lineage. The flesh of God would be David’s offspring. This Son of David would be the visible image of God reigning on God’s throne forever. This Son of David would bring the eternal kingdom of God on earth.

The angel Gabriel was sent to Mary to tell her that her son would be the Son of David.

"And the angel said unto her, Fear not, Mary: for thou hast found favour with God. And, behold, thou shalt conceive in thy womb, and bring forth a son, and shalt call his name JESUS. 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: and he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end." - Luke 1:30-33

Jesus established the kingdom of God on earth by the Gospel – by His death, burial, and resurrection. We become part of the kingdom of God by our new birth of water and of the Spirit (John 3:5, Acts 2:38, 1Corinthians 15:1-4, Romans 6:1-4). The Son of David whose throne God would establish forever, “shall build an house for my name.” The angel revealed God’s name as JESUS. The child Mary would give birth to would be the house for His name. We see this as the body of Christ.

"Jesus answered and said unto them, Destroy this temple, and in three days I will raise it up. Then said the Jews, Forty and six years was this temple in building, and wilt thou rear it up in three days? But he spake of the temple of his body." - John 2:19-21 (1 Corinthians 6:19, 2 Corinthians 6:16, James 2:7)

"To wit, that God was in Christ..." - 2 Corinthians 5:19

"And the multitudes that went before, and that followed, cried, saying, Hosanna to the Son of David: Blessed is he that cometh in the name of the Lord; Hosanna in the highest." - Matthew 21:9 (Matthew 22:41-42, Matthew 12:22-23).