I can only speak from a urban rural divide perspective, but it's largely because for a long time neither party really cares for anyone living outside of big cities. For the Democrats they always seem to consider this something to be prideful of, and always seem to have a disdain whenever they have to interact with people in rural areas. For the Republicans, they also have the same disdain, but they also try to keep it hidden, and go a step further and imply that they'll do everything they can to spite the other.

It really comes down to wanting to spite the other team for percieved or real slights they've done.