Why did Lincoln not free slaves in the beginning of the civil war?

At the beginning of the war, Lincoln's main goal was to preserve the Union, and therefore he wanted the Southern states to call off their secession. He felt he would never get them to rejoin willingly if he abolished slavery, which was one of the main things the South depended on and wanted to fight for in the war. Also, some of the Border States still in the Union allowed slavery, and Lincoln couldn't risk losing them to the Confederacy, because they allowed easy access to the Ohio River.