SS is good for BLM because there is a BLM specific mechanic that favors spell speed.

BLM needs to stack Astral Fire to do more Dmg. His strongest Attack (Fire 4) does NOT create Astral Fire. Astral Fire has a fix duration of 13s. So with more spell speed there is at some time a break even point where it is possible to cast one more Fire 4 without losing Astral Fire. And THAT is the reason why Spellspeed is a good stat for BLM.

RDM does not have such a mechanic. He does not profit from Spellspeed in that way. The opposite is true: Spellspeed is BAD for Redmage. Faster casting = more casts = more mana needed in the same time. If you could gain enough Spellspeed, you would go out of mana.