I don't usually drop dramas, probably because I'm pretty picky in the first place and usually look up trailer/storyline/actors to make sure I'll will like it.
As for the dramas I've dropped, I'd say it's mostly because of the length combined with a lack of time. If I'm totally hooked on a drama, I'd watch an episode every (other) day, but if it's not thrilling enough and I allow too much time to go by. And when I get to watch again, instead of picking up the drama I already started I might be more inclined to begin a new one.
Edit: Btw, is there a way to remove dramas from your plan to watch list? I found no other way than to "drop" them :/ although logically speaking, that's not right since i never started watching them...