They did answer to this actually, and pretty early in the live letters too (at least regarding the reasons to not bring the Path Companion back, not the lore reasons).

The main reason was that they felt that with the chocobo companion, the path companion was no longer necessary, plus, they said they had preferred to substitute the path companion's participation in instanced fights with the presence of other NPCs specific for each instance (like Thancred/Papalymo&Yda/Y'shtola in the lv5 MSQ battle instance, so to speak). Because of chocobo companions and other NPCs who heal and aid you in battle, the path companion is no longer a necessity.