It's because of Job favouritism - because not every Job would have the same debuff attacks, players would start demanding only certain Jobs that can exploit that enemy's particular weakness can enter a duty at the expense of everything else. FFXI had just such a system and suffered from player discrimination against certain Jobs or Job/Subjob combinations bad, something Yoshi has been conscious of since ARR first began development and it's something he is actively trying to prevent (this is why elemental buffs and damage was also removed).

Unfortunately that has resulted in Jobs becoming effectively homogonized and mechanically little different from each other, but that's the price that has to be paid for making the game as inclusive of all players and playstyles.