Cognitive dissonance. The Dred Scott decision was much more like the Roe v. Wade decision, in that it was was bad law arguing from perhaps very tenuous reasoning. Because a person was held legally to be a slave in one state, and the master took the slave to a state where slavery was illegal, would have compelled the destination state not to be in compliance with its own legal code.
The comparison to Dred Scott is that one side tried to do an end run on the democratic process by getting the courts to decide public policy. In neither case was there anything approaching a national consensus.