Even though Rock Head is Alolan Marowak's Hidden Ability, Rock Head is not Cubone's Hidden Ability. A Cubone with Rock Head won't evolve into a Rock Head Marowak because that isn't how ability-slots work. You need a Cubone with its Hidden Ability, which is Battle Armor. Something similar to this has been asked before here, but it's a bit different so I won't flag it.
