Why is MDL Kdrama heavy?

I think it's because K-dramas get more exposure and/or most people usually start with being into k-dramas. Korean dramas get subbed faster, they get uploaded and shared faster. It's easier to find them online when compared to dramas from other countries. 

In my personal opinion (and experience) the hype first covers Korean dramas (like Coffee Prince, Princess Hours, Boys over Flower, etc) and then people slowly realize there are other countries that are potentially making good dramas as well~ since so many people initially/or still hype over Korean dramas... they exist more in reference. :D

That's just what I think.

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.