The right answer would be Not enough delegates signed it due to the outbreak of war.
The Corwin Amendment was proposed with the intention of protecting slavery from being banned in certain states.
President-elect Abraham Lincoln actually did not oppose this amendment as part of a political strategy; he was aiming to stop slavery from spreading to other states, not to ban it where it already had been stablished.
The amendment was only ratified by four states by the time the Civil War started.
As a side note, if the Corwin Amendment was ratified, it would have become the thirteenth amendment.