The Dim majority SCOTUS decision on Dred Scott said that blacks could never be citizens. Even in states where the Christian abolitionist Republicans won the argument and slavery was outlawed -- the Dred Scott decision meant blacks could never vote and, therefore, have a say in their status. The deck would always be stacked against them, including a possible return to slavery in states that had abolished slavery. Even if they never went back to slavery the blacks would forever be resigned to being less than citizens and never have constitutional rights.
Or another way to say it, just like in our time the SCOTUS decisions in Roe v Wad and Casey v Planned Parenthood removed the ability of pro-life majorities to have their way in their states and made abortion a federal argument -- Dred Scott v Sandford in 1857 undermined the abolitionist movement at the state level and made abolition a federal argument.