Clerics cannot cast Fireball
That is really the biggest advantage of Flame Strike over Fireball. Clerics not only get it later (5th) but they have no access to Fireball (3rd) at all. Flame Strike doesn't scale with the cleric's level, while Fireball does (+1d6 per level). So it would be every cleric's wet dream to be able to cast Fireball.
Magic Users get the cake here. Clerics get a candy to make up for it.
Not only that, but Fireball also comes with physics, explaining how things will combust, how the flames expand and all of that, which is absent from Flame Strike. Things like "impacts upon a material body", or "the burst will generally conform to the shape of the area in which it occurs", forces the caster to think twice before sending a fireball flying towards his enemies, which might have a high possibility of also hurting her allies.
To clarify, casting fireball in any area that couldnt fit the cubic explosion would make it expand beyond the intended area, potentially hurting allies or innocent standbys. It was quite common for magic users to cast it inside a dungeon room or corridor and incinerate themselves and their allies.
All of that is absent in Flame Strike, making it safer to cast.