The people of pre-contact Ula'ula Island could not count on a typical guardian deity for protection, for Tapu Bulu is a wrathful god who spent most of its history protecting the island's wilderness from its human inhabitants. Yet they still sought a god who would bless them, instead of a demon to beg for mercy. Mountains around the world have long been associated with gods, from Groudon and Kyogre on Mount Pyre to the globally renowned shrine on Mount Coronet. So the people of Ula'ula island looked up Mount Lanakila, found its strongest fairy pokemon, and declared it their god.
Alolan Ninetales, admittedly, does not vocalize the same sound as the other guardian deities; poets and philosophers did call it "Tapu Keokeo", but the name never stuck with the public. Nor, as we now know, does it transform the terrain around it in the manner of Alolan deities – but the only place where they were found certainly has a uniquely icy terrain, and it was simple enough to attribute this condition to Ninetales. Ice Stones were only found high on Lanakila, which was only climbed during the Sandshrew Games, so pre-contact Alolans did not realize just how common Ninetales could be.
Although the cult of Ninetales has been abandoned in modern times, for Tapu Bulu finally became a friend to humanity, perhaps there was more merit in it than modern Alolans realize. Yes, Alolan Ninetales are related to Ninetales found elsewhere, but even fire-type Ninetales are impressively long-lived pokemon, known to hang around shrines and capable of placing curses and living for thousands of years. And do not Manaphy and Arcanine also have children? Perhaps Alolans believe so strongly that each island has its own god that they forgot the others in their midst, who tower above them all.