So why is Poni Island such a shithole?

I mean with Ula'Ula we know that Tapu Village got fucked up by Tapu Bulu when they tried to build that Megamart and we can infer that Po Town has also been cursed by Tapu Bulu for the failed rebellion against it. Meanwhile all of Poni island is a fucking desolate shithole but there's no explanation as to why. The only residents of Poni island (Hapu and her Grandmother) live like some hermits who shacked up in some destroyed town. But we don't know why Ancient Poni Path is so dilapidated. Additionally there's the oxymoron of Poni island hosting Tapu Fini, who is renowned for purifying water, yet most of the island is arid.

So what's the deal with Poni island?