Ball thinks Americans simply don't agree with liberals as much as liberals think they do. That may be true. Another problem, though, is that the rhetoric progressives use is often as polarizing and uninviting to the undecided and possibly persuadable as far-right rhetoric often is.
To be blunt, articles on left-ish-leaning sites like Slate and Salon, as well as on overtly left-leaning sites like ThinkProgress, are often snide, disdainful of conservatives (and anyone else who doesn't share their opinion), and as mean-spirited as some of their far-right counterparts.
It's often said that appealing to reason and facts doesn't change people's opinions on certain issues. Well, being a self-righteous prick doesn't, either. Maybe if some of us (I'm occasionally guilty of this behavior, too) stopped being such smug, arrogant assholes, we'd do a better job of persuading people to share our opinions.