This isn't just a liberal issue, it's a political climate issue that everyone plays a part in. Liberal 'smugness' is a reaction to conservative anti-intellectualism. When their feelings are as good as your facts, there is nothing left to debate.
No matter how simple liberals try to explain things, it doesnt matter. Conservativism has stopped having substance worth debating over when it is no longer about policy, but all about taking the opposite side of every liberal stance.
Liberals are not at fault for the willful ignorance of rural voters. No one starts off being smug and condescending to people. This happens after they've tried to explain with civility a thousand times and nothing worked. When ego and pride, not empathy and understanding, are the only things of value to conservatives, liberals resort to contempt. There is nothing left but to resent these people for being pigheaded and small minded. Their world views are set in stone by fox news, and any attempt you make at showing them the truth is deemed as a personal attack.
This isn't related to the article, but I wanted to pick at the 'benefits of slavery' question.
I think it's important to acknowledge the 'benefits' of slavery, because it's important to remember who it benefitted and at who's expense. To claim that it benefits no one would be to deny the greed and callousness that spawned these human rights abuses.
Slavery in the past has brought massive advantages and benefits to many people today through the accumulation of intergenerational wealth, at the expense of minorities who are still systematically denied access to this wealth. To claim that these benefits don't exist would be to diminish the scale of issues slavery has brought, and is still bringing, to modern day.