Jesus was related to royalty
There are three ways to become a King.
- by force
- by selection
- by birth
The way for the hoped for King of Israel was to be by birth and this new King would be a descendant of the famous King David.
King David was a very important and successful King from Israel's history.
So for Jesus to be the King he would have to be related to King David.
In the gospels we have two family trees of Jesus both of which show a link right back to King David and beyond.