Sandshrew have historically lived in desert areas. But the frequent eruptions of nearby volcanoes drove the Sandshrew to abandon the desert and migrate to snowy mountains, where they took on this form. Sandshrew’s body changed to adapt to the harsh environment of the snowy mountains.
It is said that Vulpix came to the Alola region together with humans, but the Fox Pokémon moved to the snowy mountain peaks to avoid the normal habitats of other Pokémon, and thus it ended up taking on this form. These Alolan Vulpix live on high mountains that remain covered in snow year-round.
The environment of the Alola region, where strong sunlight pours down all year round, brought about this change in Exeggutor’s form. The people of Alola boast that the Alolan Exeggutor is the true form of Exeggutor.
Darwinian evolution coupled with environmental adaptation.
There is no reason to have Fire Vulpix, Ground Sandshrew, or Psychic Exeggutor on Alola. Literally none at all, and I don't know why it's even brought into question.