>>29304396To understand why cubone evolves into that you must understand why Marowak is like that in the first place
To protect its young and themselves against the many grass type in the region they evolved to have to a fire typing. The ghost typing comes from the bond they have with their trainers.
So to answer your question why Cubone evolves into Alolan Marowak it's to protect himself and the bond it creates with its trainer
