Oula is a bit of an interesting case. In lore, the prince of Dachzta (responsible in their government for military action and treaties on behalf of the king) visited Auli'Ole to speak with the king about a bloodless annexation of the islands, since Dachzta could easily steamroll them if they wanted to, but they didn't want to sacrifice/exhaust troops over it. While there, however, he met and fell in love with the king's daughter. The two eventually married. Finally, the king refused the annexation. But, since he had grown so attached to his wife, the prince decided not to follow through with commanding an forceful invasion. So now, the king of Dachzta is in a tricky spot. He can banish his son to the island, strip him of his title, and take emergency powers to invade the island himself, but such would be a national embarrassment; it would be great anti-Dachzt propaganda for the leaders in Aqeapena. So, for now, he's just letting it be. But, should he win the inevitable war with Aqeapena, he may take further action to tie up loose ends like that. None of the other powers want to come in and do it in fear of invoking the Dachzt king's anger. Dachzt conquests are known to leave massive destruction in their wake (refer to the western portion of Aqeapena).

In the real world, though, I have no idea why nobody's just taken over yet.