Why is Japan considered a Great Power, but South Korea is not?
There are different definitions of being a Great Power. The wikipedia article mixes them, which reduces the usefulness. That being said:
- There used to be times when some countries engaged in the Great Game and similar struggles for colonies and influence. No one or two stood out as superpowers, and any one could be brought down if the others ever allied against it.
- During much of the 20th century, there were two superpowers, the US and the USSR. The USSR broke up and China rose. Below that level, there are Regional Powers -- able to influence their part of the world but not much beyond. For a variety of reasons, some of these powers have global ambitions, but not on the scale of a superpower. Remember the UK going to war over the Falklands? Few other countries could have matched that distance. Or count the places in Africa where French troops are based.
- Then there is soft power, which is very much a question of perceptions.
Japan has a considerably higher GDP than South Korea, and it has been in that bracket longer. So it makes sense, for some writers of lists, to include Japan but not South Korea.
Because Korean editors probably don't know about this article.
Just formulations like:
Japan is considered to be a great power
And then link to some single article underlines "usefullness" of that page. I'm pretty sure, that if you want, you may find article (or write it yourself) about "RK is considered a great power".
So, it's up to you and wikipedia editors whom to consider a "Great Power"