[AnnGora]

Slavery was strictly forbidden in the Northwest Territory. And who devised the plan to organize the Northwest Territory?

Jefferson.

[Verginius Rufus]

Jefferson wanted slavery banned in the entire region between the Appalachian Mountains and the Mississippi (then the western border of the US) after 1800. That probably would not have been enforced since many of the whites moving into what is now Kentucky, Tennessee, Alabama and Mississippi were slaveowners, but the idea was incorporated into the Northwest Ordinance.

Many of the settlers in Ohio, Indiana and Illinois were from slaveholding states so if they had been allowed to bring slaves with them those states might well have become slave states. That would have changed American history for the worse. I think in Illinois there was a proposal to allow slavery after it became a state which was defeated by a 4-3 margin despite slavery never having been legal there.