Q: Why did it take so long for Christians to discover this if the Bible truly affirms it?
A: Certainly early Christians heard stories about Mary and Jesus that were not in Scripture—some spurious and some true—Mary’s assumption among them. Since death and corruption results from sin, it took the early Church some time (spurred on by some heresies) to develop a more complete understanding of original sin, personal sin, and Mary’s immaculate conception. Once these issues were resolved by the likes of Augustine, they could address the issue of Mary’s assumption both theologically and scripturally.
What the Church Fathers finally saw was that God had chosen Mary—as he did Enoch, Elijah, and Moses—freeing her from sin at conception, gracing her with perpetual virginity, to be the divine Word’s mother, and to share in his salvific work. Since Christ’s redemption fully freed her of sin, early Christian found the accounts of the glorification of her soul and body as the most fitting way for her to share in the fruit of that redemption.
Here the revered Mother of God, from all eternity joined in a hidden way with Jesus Christ . . . immaculate in her conception, a most perfect virgin in her divine motherhood, the noble associate of the divine redeemer who has won a complete triumph over sin and its consequences . . . [was granted] that she should be preserved free from the corruption of the tomb. (Munificentissimus Deus 40) (John Waiss, Bible Mary: The Mother of Jesus in the Word of God [El Cajon, Calif.: Catholic Answers Press, 2023], 246-47)
As with so many pop-level Catholic apologists, the author is either ignorant of, or does not expect his readers to be familiar with, Roman Catholic teaching concerning the deposit of faith and the limits of doctrinal development.