What were three reasons why the Dred Scott case was a major cause of the Civil War?

The first and most important element of the case is that
it invalidated the concept of Popular Sovereignty, whereby the citizens of each
territory might have chosen for themselves whether slavery would be allowed. Chief
Justice Taney's opinion declared popular sovereignty a violation of the Fifth Amendment.
His decision removed any possibility of compromise in the
territories.

Secondly, the decision took away from Congress
the power to deal with the slavery issue. This had been a hot button issue all along,
and it is problematic that Congress would have been able to do so; yet the decision
meant that only a constitutional amendment could end slavery. Most constitutional
scholars of the day, including Abraham Lincoln, believed that slavery was
constitutionally protected where it existed. Taney's decision removed the possibility of
Congress dealing with it in the territories.

Thirdly, his
decision institutionalized slavery within the entire United States, even though many
states had laws prohibiting slavery, and to my knowledge those laws were never changed.
If one pursues Taney's decision to its logical conclusion, then slavery would be legal
whether it existed or not. It became an iron clad right under the
Constitution.

Slavery was such a hot button issue, by
removing it from public debate except as a possible Constitutional Amendment, Taney
virtually assured that the issue would be resolved only by
bloodshed.