I thought it was because FF 2 ( Japanese FF 2 ) was sort of disappointing and FF 3 worked so they made FF 5 like FF 1 and FF 3 to see what works, the crystal formula or the FF 2/ FF 4 style.

Games back then were like Mario were the same hero saved the same Princess each time, and weren't really like sort of different due to graphic limitations, it wasn't till the PlayStation era were sequels started to get more creative.