The key point is..
There're three biggest broadband companies that control all the networks of South Korea. (
government already granted a right to them long time ago.) They've been struggling to take the priority from S.Korea's network market by making their network faster, cheaper and more secure to appeal to customers.
This is the key point of how South Korea could be equipped with such a fastest network on the land. Of course short distance and densly populated area makes it more viable.