Mid-July to end of August is peak vacation time for the entire country. It's as if literally every one has vacation. Simply, they don't need to worry about award seats because they can fill their planes with revenue.

Even on a normal weekend, going to Jeju fills up. Weekday, sure, it's different, but this is vacation time.

Korean Air also does the same thing. They block out the entire time of mid-July-August for any Delta Awards and mark theirs as peak season. It's just a Korean thing.

As far as the Chinese, they go to Jeju straight from China. Most of them aren't worrying about GMP-CJU.