I imagined it was to prevent abuse of turning in quests for lower lvled classes. Such as using a high lvl class to clear beastment quests and turn them in with a lvl 10 job. They want low lvl classes to grind low lvl content (dungeons/FATE's).

Back in the original 1.xx version there was rampant abuse of using higher lvl players grinding off high lvl mobs while low lvl players just sat there useless in party but soaking in the high lvl experience points to power level.