I would be remiss to avoid pointing out that it was first and foremost a plot convenience; it was a way to enmesh the stories of players returning from 1.0 and new players in such a way that only a few scenes needed alternate versions rather than having two entirely separate scripts. More than anything, it was (hopefully forgivable) pretense to treat Legacies like newbies and get away with it (aside from a few important shout-outs and recollections, here and there).
But let's get down to what those excuses were.
When in doubt, blame aether. Aether sickness on its own is pretty bad; being around dense concentrations of it for too long is, like, super bad for you.Originally Posted by "Encyclopaedia Eorzea
But let's try to be a little more convincing than, "Aether, ergo amnesia." That's the ultimate cop-out, right?dizziness, paranoia, temporary paralysis, apoplexy, jaundice, blood in one's stoo─ahem, and other unpleasant side effects
After Bahamut broke the attempted sealing spell, Louisoix sent a party of adventurers into a rift beyond time and space, trusting that they would return and continue what he had begun. If you look closely when that spell is cast, you can see the symbol of Althyk, the Keeper, surveyor of change and space and god of time.
Remember that these magicks were (allegedly) most powerful magicks ever attempted. Perhaps casting someone into a rift with a spell backed by twelve deities fed by thousands of prayers utilizing a Key (Tupsimati) to summon enough prayer-infused aether to create a Dalamud-sized conjuring warps those transported persons' proper place in four-dimensional spacetime, which Alexander's story arc suggests is a thing in this cosmos. Perhaps it left a Party of Light-sized hole in the fabric of reality, and thus in memory of it, right? <waves hands mystically>
But mostly plot convenience.