if alola is such a harsh environment for both cubone and marowak, then why is it that only marowak evolved to become a fire type

Alola isn’t so much a harsh environment as it is a different environment from the other regions where cubone and marowak thrive. More specifically, cubone are largely unaffected by their Alolan environment in all ways except the circumstances of their evolution. You see, most cubone have latent spiritual inclinations, possibly due to their natural passions and the lifelong sense of grief that tends to be inherent for their species. Under normal circumstances, however, they evolve into ground-types to anchor themselves more to the physical world, but upon death, even marowak are more inclined to release spiritual energy. (There was one such instance of a violent haunting at Pokémon Tower, caused by the spiritual energy released by a recently deceased marowak.)

The region of Alola, however, is saturated with a lot of spiritual energy itself, largely due to the presence of the tapu. While cubone are still born ground-types (as that’s their natural state of being, spiritual tendencies aside), they soak up the spiritual energies like a sponge. So in an environment that’s already high in that type of energy, a cubone will incorporate some of what they’ve soaked up into the only major physiological process they go through: evolution. Consequently, that energy drives them to gain the ghost element when they take the form of a marowak. This is why all cubone will evolve into an Alolan marowak, even if they were originally born in a completely different region.

As for why Alolan marowak gain the fire element as well, this may have to do with the vulnerability of a cubone during that physiological shift. As in, they’re already forcing themselves to change elements, so the normally tropical environment causes cubone to incorporate fire into the genetic makeup of their evolution. If, however, a cubone is raised primarily on the slopes of Mount Lanakila (a snow-covered part of Alola, where ice-types such as vulpix’s and sandshrew’s Alolan variations thrive), then the change becomes more of a conscious effort than a subconscious one; the extreme cold of Lanakila’s environment forces that cubone to adopt the fire element to cope.