It was to try and rebuild Georgia after the civil war. Lincoln recognized that the Emancipation Proclamation had to be followed by a constitutional amendment to guarantee the abolishment of slavery. The 13th amendment was passed at the end of the Civil War before the Southern states had been restored to the Union and should have easily passed the Congress.
Basically. They had the biggest ego ever. The North knew that they had the more resources and the more superior military force. So they basically underestimated the South. Also it was due to the North's military generals. They didn't lead the troops the right way and all in all, that led to early Confederate victory. The South had the superior military leadership against the North.
