The first thing to understand is that even in the North, outright opposition to slavery – in the sense of wishing it to be abolished immediately – was a minority position for a very long time. Even people who personally opposed it, who wished that it did not exist, and thought it to be inhumane, did not necessarily believe in abolishing it or become activists. For many people, it just was not a priority. For this reason, many scholars draw a distinction between “abolitionism,” what we think of as wanting to end slavery, and “anti-slavery,” which is an opposition to slavery, but not necessarily wanting to do anything about it. Opinion polling of the kind we know today did not exist in the 19th century, but it appears that a fairly broad swathe of northern public opinion fell into this latter category. Anti-slavery policies could include things like blocking the expansion of slavery in the western territories, obstructing the enforcement of fugitive slave laws, ending slavery in DC (a territory and thus under federal jurisdiction), and deporting freed slaves out of the country (usually to Liberia or somewhere in the West Indies). This position was actually widely held by northern and even some southern white, and even Abraham Lincoln subscribed to it, although how committed he was to it is a topic of some disagreement. Bear in mind also that northern states didn’t just end slavery once and for all within their own borders. They often used gradual manumission laws that made sure nobody would become enslaved in the state again after a certain date; anybody already enslaved may not ever earn freedom. Connecticut, for example, did not completely abolish slavery until 1848, and there were still slaves living in New Jersey at the outbreak of the Civil War. Even in states where slavery was illegal, free black people were not necessarily enfranchised, and some free states passed black laws to prevent free blacks from living there. Actual abolitionists were considered by many, even those who opposed slavery, to be dangerous radicals.

To understand why, we have to look at what was binding the Union together. I’ve capitalized it for a reason. Antebellum Americans placed a very strong premium on states, North and South, Slave and Free, remaining united together to an almost religious degree that is hard to explain today. This wasn’t centralist nationalism. This was a belief that the distinct states, each retaining its sovereignty and customs, unified together in a federal system, preserved a free and prosperous society. Even white northerners who did not approve of slavery knew that it was integral to the white South’s economy and way of life. In this context, anybody calling for radical action to end slavery threatens the fabric of the Union, someone who would bring about “disunion.” To the political center of the day, in fact, abolitionists and southern nationalists – often called “ultras” in the parlance of he day – were two sides of the same coin: irresponsible partisans who imperiled the legacy of Washington and Jefferson for the sake of petty regional interest. Implicit in all of this, of course, is the belief that the United States was fundamentally a white republic. Black Americans, even in the North, were commonly perceived as being less capable of self-government, both in the personal and political sense. Anything that threatened the integrity of the Union for free whites was potentially a threat. Even those sympathetic to the goals of abolitionists would not support them full-throatedly.

There is also the procedural-constitutional aspect to keep in mind. Americans in the 19th century had a much firmer sense of division between state and federal powers than we do now. It wasn’t that not everybody wanted to help free slaves in the South, it was that they did not believe the US government could constitutionally do anything about it, as the enumerated powers in Article 1 say nothing regarding slavery where it already exists. This was why a great deal of political activism focused on fugitive slave laws, slavery in DC, and slavery in the territories: the argument for federal jurisdiction was not difficult to make. Some antislavery northerners even envisioned a future in which they could block expansion of slavery into the west, and then build Republican Party organizations among white southerners within their own states, and kill the institution from within, like a “scorpion’s sting.” More radical abolitionists like Lysander Spooner and William Lloyd Garrison may not have had many such procedural scruples, but a great many white northerners did.

Many white northerners, therefore, did not place a tremendous premium on ending slavery. It either wasn’t much of a concern for them, they didn’t think they could do much about it, or they feared the consequences of trying. It didn’t help that in many cases, there was little daylight between the racial views of a white northerner and a white southerner. Supporting a true racial equality rooted in the death of slavery was not a commonly held position. You asked originally why northerners might not oppose slavery. One might just as easily ask why they would.

As you’ve probably gathered, this is a complicated topic, and I’m only touching the surface. Please let me know if I can clarify anything.

Readings

James Oakes, The Scorpion’s Sting: Antislavery and the Coming of the Civil War

James Oakes, Freedom National: The Destruction of Slavery in the United States

James McPherson, Battle Cry of Freedom: The Civil War Era. This is a longer work that touches on a lot of things beyond your question, but it’s the rare book that is both good scholarship and an engaging read.

John Patrick Daly, When Slavery Was Called Freedom: Evangelicalism Proslavery, and the Causes of the Civil War

Elizabeth Varon, Disunion!: The Coming of the American Civil War, 1789 – 1859

Paul Nagel, One Nation Indivisible: The Union in American Thought, 1776 – 1861

Paul Nagel, This Sacred Trust: American Nationality, 1798 – 1898

Paul Goodman, Of One Blood: Abolitionism and the Origins of Racial Equality