Same reason why stoneskins don't stack: Two White Mage stoneskins on a 10k HP warrior is 3600 points worth of damage. Two adloquiums on the same tank is good for ~2500 points worth of damage (up to 8000 with healing buffs up)

Having multiple HoT spells doesn't save you from getting killed instantly (Death sentence, Blighted Bouquette, Raven's beak etc.) but having two stoneskins or two galvanized (especially when it crits) allows you to go #yolo on too many things it would break mechanics.