Another factor, said Ornstein, is the increasingly conservative Republican party. There is little room for ideological conversation in the GOP. Moderates these days tend to be Democrats.
At this point in history, Democrats tend to be pragmatists and flexible, whereas Republicans tend to be ideologically driving and deviation is simply not tolerated. Some might point to the GOP's use of the war metaphor to demand loyalty to the cause, others may call it Democrat weakness, but either way, it points to the GOP's unwillingness to be bipartisan rather than the DNC's leaving the GOP out of discussions.
Ornstein goes on to talk about changes in election strategy, keys in a 24/7 news cycle with pundit driven "news" as key factors for both sides that are also partially responsible.