From Archbishop Prendergast's blog:
But why is the Queen of the New Covenant the mother of the King and not the wife of the King, as so many queens have been through the centuries? The answer lies in the biblical institution of queenship in Israel.
Beginning in the time of Solomon, the Davidic monarchs of Judah imitated their Near Eastern neighbors by reserving the office of queenship to the mother of the king. This, in part, was a practical decision in a world where distinguished and wealthy men commonly possessed multiple wives. This meant that the king’s mother was not simply honored in a stately way, but she was a royal court official, an actual government figure who often wielded significant authority in ancient Oriental kingdoms.
Things were no different in Israel. The queen not only wore a crown (Jer 13:18) and had a throne at the right hand of the Davidic king (1 Kgs 2:19), but she was revered by the king himself (1 Kgs 2:19), who was accustomed to fulfill her every request (1 Kgs 2:20). Among other things, this made her a powerful advocate on behalf of the people (1 Kgs 2:13 19).
This background is important when we read the NT, for Mary is the mother of Jesus, the royal Messiah (Matt 1:1 16) who was destined before his birth to sit on David’s throne (Luke 1:32 33; cf. Acts 2:30 36). In other words, it is the Davidic kingship of Jesus that establishes the maternal Queenship of Mary.