That was a decision by Squaresoft's translator, Ted Woosely. The whole spell names translation was done because he figured he'd simplify spell names for space reasons. Holy became "Pearl" because there were white, pearlish objects, and it was the only holy-based attack in the game. Don't blame Nintendo just because you don't know why things were named like they were. Those were limitations of Square's localization process, and they carried over into Final Fantasy VII, in the most part.