Looked it up.
Apparently there's a freaking CURFEW on PLAYING VIDEO GAMES over there.
Microsoft, rather than get embroiled in what is surely a legal mess with a very bizarre and hard to enforce law, simply decided to up the age rating, so that only those who arent affected by the curfew can play it. Cant blame them, really. This way, there's no liability on their part.
Now, that being said, reading a bit more about that curfew and knowing how South Korean gamers have tended to approach some PC games (read: REALLY obsessively) it's actually a measure intended to protect the health of teens and adolescents. To a degree, I can understand this, sorta. I cant even overstate just how incredibly obsessed-over some games are over there, particularly something like Starcraft. Minecraft itself already has addictive qualities (which is what happens when you make a sandbox that can do pretty much anything).
So, it's not that it's a proper age-rating based on maturity (it's freaking Minecraft, of course it's okay for kids)... it's that it's a potentially very addictive game, and very specifically, a PC game (console and phone games are not affected, which makes no sense whatsoever) in a region where video game addiction is common.
It would seem that many people are advocating for this law to simply be removed. Extremely hard to enforce and it's also something that the freaking PARENTS should be doing (like, seriously, pay attention to your bloody kids, not your stupid phone!).
Anyway, that's just some of what I read about it.