That was an amendment that Lincoln promoted and worked hard to have passed. That it was quickly ratified was at least in part a posthumous tribute to him.
It's clear to all who understand the subject that a presidential proclamation couldn't have freed all the slaves. Such an action could only be constitutionally justified as a war measure. An amendment would be necessary to completely abolish slavery.
That yankee education isn't all it is cut out to be, is it?
Neither is second-rate Confederate propaganda.
Even after the Emancipation Proclamation, two more years of war, service by African American troops, and the defeat of the Confederacy, the nation was still unprepared to deal with the question of full citizenship for its newly freed black population. The Reconstruction implemented by Congress, which lasted from 1866 to 1877, was aimed at reorganizing the Southern states after the Civil War, providing the means for readmitting them into the Union, and defining the means by which whites and blacks could live together in a nonslave society. The South, however, saw Reconstruction as a humiliating, even vengeful imposition and did not welcome it.
After the Civil War, with the protection of the Thirteenth, Fourteenth, and Fifteenth Amendments to the Constitution and the Civil Rights Act of 1866, African Americans enjoyed a period when they were allowed to vote, actively participate in the political process, acquire the land of former owners, seek their own employment, and use public accommodations. Opponents of this progress, however, soon rallied against the former slaves' freedom and began to find means for eroding the gains for which many had shed their blood.
http://memory.loc.gov/ammem/aaohtml/exhibit/aopart5.html
Well Ms. "I have gone into this program with a BA in history", Congress passed legislation ending slavery in D.C. on April 16, 1862. It could do this because Article I, Section 8, Clause 17 of the Constitution gives Congres the power to "To exercise exclusive Legislation in all Cases whatsoever, over such District (not exceeding ten Miles square) as may, by Cession of particular States, and the Acceptance of Congress, become the Seat of the Government of the United States..."