The correct answer is:
Because he changed the goal of the war to freedom and consecrated all the dead on the battlefield to this cause, not just the Union soldiers.
The Gettysburg Address is considered a seminal moment in American history because of its powerful and inclusive message. In the speech, President Abraham Lincoln redefined the goals of the Civil War, from preserving the Union to preserving freedom for all people. He acknowledged the tremendous sacrifice made by the soldiers who died on the battlefield, both Union and Confederate, and consecrated their memory to the cause of freedom.
This address is considered magnanimous because it focused on the larger ideals of freedom and democracy, and not just the narrow interests of one side or another. The speech also helped to bring the nation together during a time of great division and strife.