Although the gods of each Alolan island are indeed guardian deities, they have understood this task in different ways. Tapu Koko and Tapu Lele protect the peace of Melemele and Akala, while Tapu Bulu has historically served as guardian of Ula'ula's wild pokemon, only recently coming around to accepting human presence (and encroachment) therein. Poni Island, on the other hand, is a famously remote and tranquil place, and Tapu Fini has long striven to keep it that way – by making sure no outsiders can reach it!
Poni Island lies at the edge of the Alolan island chain, and is further from Ula'ula than Melemele is from Akala, but this is not sufficient to explain its low human population; Alola itself is far further from any other landmass on Earth. The ancient Alolans, after all, were expert sailors; had this been otherwise, they would never have navigated even a single ship to Poni Island. For the waters around Poni are among the most treacherous on Earth, and the sea floor around it is littered with the wrecks of ships that sunk in Tapu Fini's storms.
Even today, only skilled captains will dare to sail the Ula'ula-Poni route, and those who board that ferry must first sign a waiver informing them that they do so at their own peril; earlier attempts to reach the island through the air have suffered from both heavy fog and Hydro Pumps knocking them out of the sky. Curiously, however, Poni Island's inhabitants often support themselves through fishing, and (provided they do not bring outsiders with them) may come and go from the island with ease. But the island is not wholly isolated; the world's strongest trainers see this journey as a test, and surviving Tapu Fini as the first step towards conquering the legendary Battle Tree!
