Why is that? The guy was a political activist out to stir up trouble, and he succeeded. Liberals are still doing this exact sort of thing today.
Woodpusher can give you a great deal of insight into the actual case of Dred Scott. I think he has written excellent commentary on the point, and perhaps he will do so again.
Why focus on the Massachusetts representative? He did not chose to take the case. He did not write the opinion. He did not conspire with other justices and President Pierce about the case. Pierce was a Democrat and a staunch anti-abolitionist.
The case was somewhat unusual, and Taney had to stretch a bit to take it.
Pierce was a drunk. Many consider him one of the worst American presidents.