With guerrilla warfare you have, essentially, no supplies, no support, and no concentration of force, because any sizable cache of supplies will be bombed out from under you or seized, support (in this case referring to armor and air support) will be bombed before it ever gets to the front lines (in a modern conflict against a power that essentially defines air superiority) or left without fuel and ammo (see: supplies), and you can't concentrate your forces because a massed number of troops presents a nice target for the aforementioned bombing.
Then you have the nice little bonus of pissing off the occupiers to the point where they start slaughtering civilians in retribution, destroying a country utterly (see: every counter-guerrilla effort before the nineties)...
Of course, in this day and age, you can't just napalm every village you find and expect to get away with it, but there's been a good deal of advancement in the field of turning natives against the guerrillas with humanitarian support... In the case of Korea, it would help a great deal that it's other koreans rolling in instead of foreign soldiers that speak another language.
Now, you can make yourself a nuisance with guerrilla tactics, but you can't win a war, because if you've turned to guerrilla tactics, you've already lost, and are just clinging painfully on in the hopes that you'll still exist when it's all over. Which, in the case of Korea, will never happen, because the south taking the north would result in a permanent military occupation, hence the guerrillas can't just wait them out like, say, the taliban is trying to do (it can't mount a meaningful military resistance, all it can hope for is to still be a viable power when the US ups and leaves, and given that the whole point of being in afghanistan in the first place was to wipe out the taliban as a viable power...).