Finally the Immaculate Virgin, preserved free from all stain of original sin, when the course of her earthly life was finished, was taken up body and soul into heavenly glory, and exalted by the Lord as Queen over all things, so that she might be the more fully conformed to her Son, the Lord of lords and conqueror of sin and death.
-- Pius XII, Munificentissimus Deus
Mary was exempted from bodily corruption because, by an entirely singular privilege, she completely overcame sin by her Immaculate Conception. Jesus ascended to Heaven by His own power as Creator and Lord. Mary was taken to Heaven by the power of God, raised aloft by grace, not by nature.
Pope Pius showed the relation of the Assumption to the Immaculate Conception: "For these two privileges are most closely related to each other. Christ has overcome sin and death by His own death; and one who is reborn in a heavenly way through baptism has, through Christ Himself, conquered sin and death. However, in accord with His general rule, God does not wish to grant the full effect of victory over death to the just until the end of time shall have come.... Yet God wished that the Blessed Virgin Mary be exempt from this general law. For she, by a completely singular privilege, conquered sin in her Immaculate Conception, and thus was not liable to that law of remaining in the corruption of the grave, nor did she have to wait for the end of time for the redemption of her body" (AAS 42. 754).