There are probably several reasons. I think one of them involves a desire to increase devotion to the Blessed Virgin, in order to draw people back to God and the Church. In my view, secularism has been swelling in pockets of the West at least since the French Revolution (and probably going back long before), and those pockets of secularism have really ballooned into huge, dominant blocks. I think the emphasis on the Virgin Mary has kept a lot of the faithful in the fold in the face of all of this.
In
this article, Catholic author Kenneth Howell cites several reasons why the Church proclaimed Mary’s assumption as a dogma. One is, “in the year 1950 the world was still reeling from the devastating effects of World War II.” Increased devotion to the Virgin could be healing. Pope Pius XII mentioned another reason: “the illusory teachings of materialism and the corruption of morals that follows from these teachings threaten to extinguish the light of virtue…” “[But through the proclamation of the assumption] all may see clearly to what a lofty goal our bodies and souls are destined.”
source
So those are three possible reasons: devotion to the Virgin could combat secularism, provide healing after World War 2, and remind us of the exalted plan God has for both the body and the soul.