My only thought on this (for this spell and several others) is that because the 5E14 Wizard was given the ability to cast rituals in their spellbook even if they hadn't prepared them for the day... it would increase the amount of magic the Wizard would take care of for the party at the detriment of the other classes. The other classes with Zone of Truth would virtually never use them if there was a Wizard in the party, so there would be no reason for those classes to prepare the spell (since they'd have to have it prepared in order to cast it as a ritual themselves) making those spells on those other class's spell lists almost superfluous.Honestly, zone of truth should be a ritual, and I'm really not sure why it isn't.