Why was the north against slavery in The US?

Answer 1

It wasn't particularly.

It was against slavery in the North, mainly because the new industrial factory managers could not use it. They wanted free, mobile skilled labour. There was also the Abolitionist movement - powerful, but not very numerous. In 1861, most Northerners found slavery acceptable, as long as it stayed in its traditional homelands.

It was the creation of new slave-states that was the issue, partly because of the political balance between North and South in Congress, and partly ethical - the feeling that slavery was a necessary evil that should not be extended.