It's because Korean dramas are subbed quickly and tend to be more accessible to fans.
I started with Japanese dramas and watched only them for years, but I speak Japanese so don't need subs. It would be a lot harder to find the dramas I love (older Japanese ones) if I needed to rely on subs.
Nowadays that more Korean dramas have plots that aren't centered just around romance, I watch a lot of them too, and try to expand into dramas from other countries as well. The reason I personally haven't watched many Chinese, Taiwanese, or Thai dramas despite wanting to is because I lose interest in long series -- both in terms of number of episodes and episode length.