Why did some theologians believe that the Immaculate Conception conflicted with Christ’s universal redemption?

Some theologians, including St. Thomas Aquinas, believed that the Immaculate Conception conflicted with the doctrine of Christ’s universal redemption. The concern was that if Mary were preserved from original sin from the moment of her conception, she would not have required redemption by Jesus, thereby undermining the belief that his grace and redemptive work were necessary for all of humanity.12

Blessed John Duns Scotus addressed this challenge by arguing that Christ’s redemptive grace could be applied to Mary in a preventative rather than a merely curative way. By preserving her from sin before it could reach her soul, Christ performed a more perfect act of mediation. In this view, Mary’s freedom from sin was not an exception to universal redemption but a result of it, made possible solely by the merits of Christ.123