Why does the bible refer Jesus the son of David?

Jesus is called the Son of David fourteen times in the New Testament. Twelve of these times, Son is capitalized. Two times it is not (Matthew 1:1The book of the generation of Jesus Christ, the son of David, the son of Abraham.
See All...; 12:23
See All...). In the Bible, son can have several meanings. So, although it can refer to the biological and immediate son of an earthly father, it can also have other meanings.

For example, to be a son can mean to be a direct descendant. Joseph, the husband of Mary, is called the "son of David" in Matthew 1:20But while he thought on these things, behold, the angel of the LORD appeared unto him in a dream, saying, Joseph, thou son of David, fear not to take unto thee Mary thy wife: for that which is conceived in her is of the Holy Ghost.
See All.... That means that he was a direct descendant of David and so David was a many-greats grandfather to him. This is true of Jesus through Mary (the genealogical line of Luke 3). Jesus was a son of David through her.

However, the New Testament Jews knew the title, "Son of David," as another title for the Messiah. Jesus mentioned that the scribes taught that "Christ is the Son of David" (Mark 12:35And Jesus answered and said, while he taught in the temple, How say the scribes that Christ is the son of David?
See All...). When Jesus healed a man who was both blind and deaf by casting an unclean spirit out of him, the people said, "Is not this the son of David?" (Mathew 12:22-23
See All...). They were identifying Jesus as the Christ, the Messiah who was promised to come. Several reasons then might be given for this title being applied to Christ.

1. The Messiah was to sit on the throne of David (Isaiah 9:6-7 [6] For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace. [7] Of the increase of his government and peace there shall be no end, upon the throne of David, and upon his kingdom, to order it, and to establish it with judgment and with justice from henceforth even for ever. The zeal of the LORD of hosts will perform this.
   See All...; Jeremiah 23:5Behold, the days come, saith the LORD, that I will raise unto David a righteous Branch, and a King shall reign and prosper, and shall execute judgment and justice in the earth.
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2. Therefore, it was necessary that the Messiah was born in the direct line of David (Psalm 132:11The LORD hath sworn in truth unto David; he will not turn from it; Of the fruit of thy body will I set upon thy throne.
   See All...; Acts 13:22-23 [22] And when he had removed him, he raised up unto them David to be their king; to whom also he gave their testimony, and said, I have found David the son of Jesse, a man after mine own heart, which shall fulfil all my will. [23] Of this man's seed hath God according to his promise raised unto Israel a Saviour, Jesus:
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3. This promise of a king to sit on the throne of David was fulfilled in Jesus Christ (Luke 1:31-33 [31] And, behold, thou shalt conceive in thy womb, and bring forth a son, and shalt call his name JESUS. [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: [33] And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end.
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And, since Jesus was the Son of David, He was also the King of the Jews (Matthew 2:2Saying, Where is he that is born King of the Jews? for we have seen his star in the east, and are come to worship him.
See All...; 27:11
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