Anyone knows?

Well, not enough or not interested enough.
Whatever the reason, if a drama stays undone or half-done for a year or two, it is obvious. And sometimes viki sends a paid subber to finish it.
Sometimes the CM can ask for help as well. For instance, sometimes there is only a European license and most of the Ko-En subbers are in the US, so they cannot access the video (and they refuse to use vpn as a matter of principle). So the CM asks viki to send reinforcement.

Hmm, I see.