Hypothesize about why slavery was outlawed in the northwest territories

The Northern states probably banished slavery before the South did because they had no real good use for them. Most of the labor in the North before the Industrial Revolution was things like ship making and fishing. They didn't have real work for the slaves that was very hard like in the South. The South has plantations and those were hard to deal because they had a lot of things to do at once and also because it was a very physically demanding labor. They needed certain people who they didn't need to pay and were seen as less to do them. That is probably why.