Maple Syrup and Freedom
The original slave crop of the revival of slavery in Western Civilization wasn't cotton; it was sugar. I suspect that may people were reluctant to oppose slavery simply because they had a sugar habit and were unwilling to be hypocrites.
If that's the case, New England may have become a center for opposition to slavery simply because they used maple syrup instead. (Ob Thanksgiving: Maple syrup was almost certainly available at early Thanksgiving celebrations.)