Even though slavery may not have been equal in all of the original 13 slave states, when the time came everyone of the 13 states voted to enshrine slavery into the United States Constitution.
Some of the northern states may have even had qualms about voting to enshrine slavery into the constitution because of a deep belief that human bondage was morally wrong. Still, the original northern states all voted not just to include slavery in the constitution, but to include an amendment process that would make the peaceful abolition of slavery all but impossible short term.
But the northern states voted for slavery for a good reason: it was in their economic and political best self-interest.