Probably because Fire III and Blizzard III function differently than Fire and Blizzard.
Fire and Blizzard require you to actively use Transpose to switch between Astral Fire and Umbral Ice, and they only grant 1 stack per use up to 3. Fire III and Blizzard III switch between the two for you without having to Transpose, and automatically grant you 3 stacks of each. While they could have adjusted for this, they chose for some reason not to. Perhaps a BLM main can provide more insight on that, but all I can say is the two function differently, where as a lot of the other upgrade spells were more or less the same (did the same thing with slightly higher potencies/mana costs like Stone > Stone II > Stone III > Stone IV; Malefic I > Malefic II > Malefic III; Ruin > Broil > Broil II; etc.). Also, Fire is the only Fire than can proc Firestarter, so I can see why they kept Fire for that very reason. Blizzard is just sort of there still, but could be they kept it not only because of different function, but to have the Fire - Blizzard pair.
When I think about it, the other similar spells they kept, they kept because they function differently. For SCH, Ruin was upgraded to Broil, but they kept Ruin II, the insta-cast (can’t speak for SMN on this, but I’m pretty sure Ruin II and Ruin III are still separate, because they function differently from one another—one is insta-cast for use when having to move, the other has a cast time). Same for Aero I > Aero II and Aero III: one is single-target, insta-cast; the other AOE with a cast bar. Different functioning.
