It was a moral imperative for much of the North. Lincoln only barely scraped out the Republican nomination. His main opponent was William Seward who was a “radical” abolitionist. Had Seward won the nomination, there may have been some fracturing of the newly formed Republican party. So while there was indeed a portion of the population who felt the complete abolition of slavery was too far, a huge chunk agreed with Seward. In particular, his own wife, Francis Seward. She abhorred slavery and I urge everyone to read her writings upon the subject.