I recently learned that a mother always retains some cells from her children she carried. It was in the context of women recovering from abortion and physical realities that they might not even realize that they are dealing with. Anyway, with that in mind, it makes sense that the Blessed Mother would be preserved from death.
It's not that she couldn't die, or wouldn't have accepted it. It just seems right that she who nurture the Word, whose womb housed Him, lips kissed Him, breasts fed Him, hands wiped His tears, who was kept from sin by the grace of her Son, it just doesn't seem right that her body would endure the consequences of that which He protected her from.
"...it just doesn't seem right that her body would endure the consequences of that which He protected her from."
Indeed and as +Gregory writes:
"Or, if while yet three years of age and not yet possessing that super- celestial in-dwelling, she seemed not to bear our flesh as she abode in the Holy of Holies, and after she became supremely perfect even as regards her body by such great marvels, how indeed could that body suffer corruption and turn to earth? How could such a thing be conceivable for anyone who thinks reasonably'?"
But she did indeed die:
"Hence, the body which gave birth is glorified together with what was born of it with God-befitting glory, and the "ark of holiness" (Ps. 131:8) is resurrected, after the prophetic ode, together with Christ Who formerly arose from the dead on the third day. The strips of linen and the burial clothes afford the apostles a demonstration of the Theotokos' resurrection from the dead, since they remained alone in the tomb and at the apostles' scrutiny they were found there, even as it had been with the Master. There was no necessity for her body to delay yet a little while in the earth, as was the case with her Son and God, and so it was taken up straightway from the tomb to a super-celestial realm, from whence she flashes forth most brilliant and divine illuminations and graces, irradiating earth's region; thus she is worshipped and marvelled at and hymned by all the faithful."