Because of the little fact it was
a story element in the main scenario, that it was a spell hidden within the Heart of Sabik, the Ultima Weapon's power core which Lahabrea used to destroy the player's 'Shield of Light' (the barrier that Hydaelyn's Blessing shielded the player with) and indirectly the Praetorium - thus because when the Ultima Weapon was destroyed, the spell was kinda lost/destroyed along with it after that?
Thus because of that, Ultima will
never be castable by players - think of it like FFVII where Holy and Meteor were also tied into the story and could never be used by the player either in battle. Hells, even FFXI never had Ultima available as a player-castable spell either.