The industrializing North had little use for slavery, which was unprofitable for manufacturing. Unending streams of European immigrants to the North (in large part sourced by enclosure acts like those in Scotland in the early 1800s)[3] provided a huge reserve army of labor cheaper than what it would cost to buy, care for, guard, and educate enslaved people.

Even agricultural regions in the North were ill-suited to slavery, as wheat crops were seasonal and enslaved people would need vital needs provided year round. In the North, slavery wasn’t abolished on a moral level, rather it was simply less economical than “free” labor from a reserve army of labor.

The Civil War began as a war against separatism, and Lincoln did not initially want to abolish slavery. Later, when slaves and freedmen entered the Union army, they turned the war into a revolution against slavery.[5] After many enslaved people had already escaped to the North, Lincoln issued the Emancipation Proclamation in 1863 and allowed them to serve in the Union army.

Link

It’s a pretty neutral ground really since the other half was fighting at the moral low point, and then they got to dictate terms for the next century