Why was the north against slavery in The US?

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.