Let's remember that several western countries do not want a united Korea either.
Koreans have shown already that they sometimes still think of themselves as one people, with north and south koreans being friendly toward each other and wanting a reunion of the country (the way of the reunion that is the disagreement).
North Korea has the biggest standing army of the world.
South Korea also has a huge army.
Also both of them have huge amounts of military equipment.
And finally, Korea in general (both from north and south) and very belligerant, or was at least, in the past, Korea history is filled with wars that they caused, and crazy conquest attempts.
Other asians are still very wary of ethnic Koreans, in Japan for example it is known that while there are several non-japanese citizens, ethinic Koreans rarely get citizenship.
The united Koreas, maybe will just awesomely advance the world technology... Or maybe they will start another crazy war and attempt to conquer Japan or China.
There are several countries that don't want to risk the second option, and thus every time one Korea is losing advantage in the stalemate, that Korea gets some sort of clandestine or official help, in a way to force the stalemate to continue, and the biggest army of the world divided in half and turned into itself.
I am not saying I agree or condone that, but this is how it works.