It may seem odd at first glance, but that's because knowing what goes on in the subsequent decades we take the South as our model of how a slave state behaves. They're for slavery in perpetuity and slavery above all other concerns, or near enough to it. That's probably true of the South in general, if not every enslaver living there, by the 1790s. But while the South is consolidating and doubling down on its slave system, the North is largely not. The 1780s are when we see the first emancipation laws and court decisions. Nothing like that happens south of Pennsylvania or the Ohio until the Civil War intervenes.

That doesn't mean it's all sweetness and light up North. Emancipation there frequently meant enslaved people sold South in advance of the day of freedom. (It was illegal, but it happened anyway.) Freedom could also mean black Americans forced out of their previous skilled trades and into more menial labor, as they had trouble getting or keeping patrons when working for themselves rather than for a white owner. Despite freedom, they faced considerable legal and practical discrimination compared to whites. In some places, that's something that gets progressively worse as the decades go on.

Nor does any of this mean that the North didn't have any people in favor of slavery forever. In some cases the emancipation votes are quite close. (Emancipation lost in New York in the 1780s and didn't come back until 1799. New Jersey took until 1804.) In most states, the emancipation that comes is extremely deferential to the property rights and money that enslavers have invested in their slave property. That means that most enslaved people there don't see freedom for decades after. There are significant slave populations in parts of the North into the 1820s and occasional movements to rescind the emancipation laws. What happens instead is that you see laws passed to free the slaves who fell through the cracks of the prior laws, but these come decades later. The last pass in the 1840s, so it's only then that you have fully slavery-free states become the norm. These laws never reach New Jersey, which has slaves on its 1860 census returns. The sectional cleavage is meaningful and significant, but it's not absolute and the North is consistently the more diffuse section with the weaker sense of identity compared to the South.

All the same, the degree to which the North is free and antislavery (and antislavery sentiment is genuine, though often underappreciated, from quite early on) creates a political space to vote for slavery bans just as it makes the same space to vote for gradual emancipation. There's a contingent for it and sticking your neck out on the cause ins't going to be political suicide.

This all speaks to how people could vote for the Northwest Ordinance, the nation's first real antislavery law. That status isn't necessarily clear at the time, though. Later generations of antislavery thinkers will hold up the law as one of their canonical texts and cite it as an important precedent for banning slavery in the territories. It's right up there with the Constitution (for mainstream, politically-inclined antislavery people), the Declaration of Independence, and abolishing the Atlantic slave trade. If the federal government could keep slavery out of the Northwest Territory, then obviously it could keep slavery out of Missouri (they lost that vote), most of the Louisiana Purchase (a concession prize that initially was taken as no more than a fig leaf for voting slavery in in Missouri but becomes of great significance in 1854, as discussed on the most recent AH Podcast), the Pacific Northwest, and the Mexican Cession. It's the wellspring of all those things, to the point of its slavery language being repeated all through. You'll see it taught like that in survey courses and it's not all wrong.

But it is partially wrong. There were slaves in the Northwest Territory, but it's not clear that Congress knew that when they voted through the Ordinance. As such, and in the absence of subsequent legislation or state action to free those slaves, we can't fairly take the Ordinance as an abolition law. The enslavers out there, mostly in future Illinois, were worried and lobbied (often with the support of the territorial governments) Congress to end the slavery ban. Congress did not deliver, but also didn't take any affirmative action against the then-present slavery when it came to their attention. This reduced the Ordinance's slavery ban to a fairly weak ban on taking more slaves into the territory.

It gets worse. The slavery ban is actually a last-minute addition to the Northwest Ordinance. The closer you look at the law and its enforcement, the harder it is to come up with a clear idea of just what the men who wrote it and voted it through really thought they were doing.

First off, the slavery section was adopted without debate. Thus we don't have clear explications of what the law meant to the legislators in the moment. The Lower South politicians who voted for the law almost certainly did not think of it as an antislavery measure, even remotely, or they'd have voted otherwise. Nor was the rest of the Ordinance redrafted for consistency with the slavery ban, so it presumes that slaves exist and will continue to exist within the jurisdiction. The main way you see this is in reference to free inhabitants, which entails the existence of unfree inhabitants. Slaves. French inhabitants are also guaranteed their property, which includes slaves. Slavery continues, in one form or another and at least in part because of that ambiguity, for decades in the Old Northwest.

The law does not threaten bondage in the South, and might even have reinforced it by removing potential competition and implicitly guaranteed slavery beneath the Ohio. Previously voting slavery out of all the territories after 1800 was on the table. It lost and the Ordinance we got is a much more modest thing that, viewed in the context of the whole of the territories, is at least a potential gain for slavery. It also includes a fugitive slave clause which grants to the South a power to recover self-stealing slaves that it had hitherto lacked completely.

The ordinance furthermore lacked any enforcement provision for its slavery ban. It doesn't place ending or excluding slavery in the hands of any clear entity empowered to pass legislation or so forth. You could sue, petition, or pass laws to protected other rights in the territory under other sections of the Ordinance, but no such open door existed for people enslaved in defiance of the law. Slaves could maybe sue, but that's a long shot given the circumstances. The Indians both had more legal protection and more means to defend their rights (up to and including war) than an enslaved person did.

It's also important to view the ordinance in light of settlement patterns at the time. We tend to see the South as the slower-moving section, but that's a later development. Kentucky and Tennessee become states in the eighteenth century. Up north, only Vermont shares that distinction. Much of the early white American population in the Northwest is actually southerners coming across the Ohio, not Yankees coming over the mountains. There are multiple reasons for that, but the main ones are raw geography (Ohio is not right next door to most of the North the same way Kentucky and Tennessee are adjacent to the South) and the continuing, somewhat greater, existence of somewhat unsettled hinterlands within the bounds of Northern states. Western Pennsylvania is a frontier, at least to a degree, up into the Washington administration. Why move all the way to future Ohio when you've got opportunities closer to home? The Ordinance does not apply to Tennessee. The Southwest Ordinance, which covers it, is a copy of the Northwest except for the slavery language. The territory south of TN is also looked at as an expansion region and there's no slavery ban there either. So the places where one would expect the earliest white expansion, and where it does in fact take place, are wide open for slavery. It's only a rump that isn't expected to grow so quickly which has a qualified, ambiguous antislavery provision in its organiziation.

Sources

Ira Berlin's Many Thousands Gone

Paul Finkelman's Slavery and the Founders, second edition

William Freehling's Road to Disunion, volume one