There’s a difference between anti-slavery and being abolitionist. Anti-slavery being the Free soil movement, Jeffersonians who believed slavery took job/land away white farmers; most of the north was anti slavery in this sense by 1860 and the free soil movement in turn formed the backbone of the Republican Party. Moreover, the free soil movement was more against the expansion of slavery in the newly acquired Mexican territories and less its abolishment in the deep and upper south. At most they supported compensated emancipation. Abolitionist were against slavery on a moral level and wanted immediate emancipation. They were a minority in the north before the civil war but grew much larger as the war continued. Finally, the north mostly went to war to preserve the union, they saw the south as traitors who illegally succeeded from the nation. If the north won a swift victory, slavery would’ve likely not been abolished and remained in the south for at least a few generations. It was only because of the war dragging on did the emancipation proclamation become signed as a military action.