There were plenty of slaves in the North, too.
You just don’t hear that much about them because it messes up the narrative.
What narrative?
Slavery in the North effectively ended as a result of the Revolutionary War. Many free blacks in the north had been free for several generations. Their ancestors had been bought as slaves in Africa early in colonial history but were treated as bond servants/slaves like the whites whose debts/sentences were purchased. They had a specified period of servitude after which they were freed. After the Revolution many former slaves in the north were sent packing, in part because they were loyalists who went with the British.