he needed a boost to make it acceptable by the public. by the end of 1862 the Union was on a victory role (esp. stopping Confederacy at Antietam) in the east and this was the right time.

you have to take into context that it only made slavery illegal in all areas that were rebelling. ie.. Maryland for ex got to keep slaves until the 13th amendment.

Lincoln had to frame the Emancipation Proclamation as not a morality issue but as a military measure necessary to defeat the Confederacy if you're average-Joe in the North was going to support it, and the beginning of 1863 was the right time for him to do so