Other than the few groups of abolitionists in the New England states the North had no truck with slavery. The modern history books would have us all believe that the South and the North locked horns over it and the heroes won and abolished it. Problem is for the South it (the expansion of slavery to the territories and new states) was one of many reasons to secede. For the North it was a non-issue. The North made war on the South to preserve the union. Period.
And what the southerners really wanted with the western territories was a place to get rid of their black population. There was never any real hope of extending the plantation system there. The land there won't support it.
I wouldn't say it was a non-issue for the north. What the republicans wanted was to deport the blacks out of North America altogether, and to keep them in the South until that could be arranged.
“The North made war on the South to preserve the union. Period.”
The North made war on the South to preserve the revenues the North enjoyed from the South. The South was not disproportionately taxed because it paid most of the federal revenues (which it did, because of all its exports), but because the vast majority of those revenues were spent and distributed up North. The South saw it is unfair that it should pay the lion’s share of the federal bill when the vast majority of those monies were spent elsewhere.