Because the pages contain relatively few Hubs, any Hub published within these categories (be sure, of course, that it is truly related) is more likely to be prominently featured. History of Japan has not yet reached the threshold of 5 or more featured Hubs, which is why it does not have a link. When that threshold is reached, a Topic Page will automatically be created, giving all its featured Hubs very high visibility on that Topic Page.
No "History of Japan" link yet! I took a lot of Japanese culture classes, so maybe I can come up with one Hub. I bet a whole lot of other Hubbers out there have visited Japan, served with the armed forces there, or have some good information about the history. Maybe we will see that link this week!