One of the most common political divides is that cities tend to be much more progressive and rural areas more conservative. It makes sense: In rural areas, population density is much lower, so people resolve their conflicts amongst themselves, and they see little impact from governmental actions. And since there’s not a lot of people movement or economic activity, new problems won’t quickly emerge, so tradition is a good proxy for the set of rules you should follow. People vote for smaller governments and more conservative parties. Conversely, in cities people cross paths with strangers all the time, so the state often needs to intervene in social conflicts, and its footprint is therefore everywhere, and people vote for more progressive parties that want a bigger administration.
Since the east of Korea is much more mountainous, it’s much less densely populated, and hence leans more conservative.