The issue being that, Fire IV is our Astral Fire phase filler spell. It's the spell we default back to when we have nothing better to cast. Just like how every other non-combo based job has their own filler skills (Healers have Broil/Malefic/Stone, RDM has Jolt/Impact, SMN has Ruin, BRD has Heavy Shot, BLU has Glower)
But... We simply don't have anything better to cast. Not in Astral Fire phase. Yes, we have stronger skills - Thundercloud proc and Foul have higher potency. But their lenience in when to be cast, combined with the complete lack of any actually worthwhile Ice skills, means that it's never necessary to cast them during Astral Fire phase.
If there was a conditionally better skill to use during AF (Such as how at levels 42-60 we have Firestarter procs to allow us to use a free, instant cast Fire III) or if there was actually worthwhile skills in UI (So that "Saving" TCloud and Foul to use during the time we're actually in that phase wouldn't be a thing) then there wouldn't be this issue of staleness with AF and the Fire IV spam.