It is because Mary conceived Jesus of the Holy Spirit, making her set apart for God in the same way the Tabernacle was set apart in the OT. The word for overshadowing the Angel Gabriel used to announce Jesus’ conception is the same used to describe the overshadowing (the glory) of the Tabernacle:
Exodus 40 [34] Then the cloud covered the tent of meeting, and the glory of the LORD filled the tabernacle.[35] And Moses was not able to enter the tent of meeting, because the cloud abode upon it, and the glory of the LORD filled the tabernacle. [36] Throughout all their journeys, whenever the cloud was taken up from over the tabernacle, the people of Israel would go onward; [38] For throughout all their journeys the cloud of the LORD was upon the tabernacle by day, and fire was in it by night, in the sight of all the house of Israel.
Matt.1[20] But as he considered this, behold, an angel of the Lord appeared to him in a dream, saying, "Joseph, son of David, do not fear to take Mary your wife, for
that which is conceived in her is of the Holy Spirit…[23] “Behold,
a virgin shall conceive and bear a son, and his name shall be called Emmanuel” (which means, God with us).
Luke 1[30] And the angel said to her, "Do not be afraid, Mary…[31] And behold,
you will conceive in your womb and bear a son, and you shall call his name Jesus… [35] And the angel said to her, "
The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy, the Son of God.
Sadly, much of this rich theological understanding was lost among our Protestant brethren after the Reformation–gradually, though, not right away, in fact not for a couple of centuries. Luther and Calvin would easily have recognized and understood this prefiguring of the Tabernacle as Mary the Mother of God (Theotokos [God-bearer] in the Greek) and the Spouse of the Holy Spirit. I hope that helps.