Because UX is both a retelling and continuation of Chi. So it makes sense for them to finally retell the events of the Keyblade War. There hasn’t been an official playable version of the Keyblade War available for the West since browser Chi was for Japanese players only, so this is the first time many will get to experience this part of the story if they haven’t been following along with translations.
Ah true, I completely forgot that we never actually got to experience it firsthand. I've followed Chi and Unchained X as well as playing a bit myself for so long that I'm thinking from a lore perspective and not from a new mobile player's POV.