A lot of dramas are not pre-produced, they're made during a certain amount of time and scripts may change according to viewers' opinions. When the drama has finished airing, actors move on to other projects. If you're producing a drama at the same time as it's airing, then you really can't work around anyone's schedules. I think it's easier for pre-produced dramas to get sequels, since you can plan ahead for them and it's easier to gather the same cast members if you don't necessarily need them available for 7 weeks or so, all at the same time, with no way to incorporate other schedules.