Slavery had basically been a thing for all of human history up to that point, and based on my discussions on HN many smart people don't believe a lot of what Adam Smith said. There are still a lot of basic economic ideas that would make people much wealthier that struggle to get out into the wild. With that perspective the near-total abolition of slavery in a century seems pretty quick. And it can't really be a social thing because it is clear from history that societies tolerate slavery if it makes sense.
And we see what happened to the people who tried to maintain slavery over that century - they ended up poor then economically, socially and historically humiliated.