Ashley Riot saved Ivalice.
According to in-game hints and the FFXII Ultimania, the most recent event in old Ivalice is Ashley Riot taking on the power of the Gran Grimoire, and with it control over the Dark. The next we know of Ivalice, it's a small town without magic, but with at least two grimoires - one bearing the Kildean Rood. Conclusion: after obtaining control over the Dark, Ashley saved Ivalice from its power, which seemed to be corrupting all magic. Either he captured it all in grimoires and left Ivalice without magic, or he led an exodus of souls beyond the Rift to Earth (which has precedent elsewhere in the franchise) and bound the memory if Ivalice into those grimoires. Whatever the case, he blessed the Grimoire of the Rift with the Rood before he departed himself. Ivalice is being assaulted by a hostile world.
The first we know of Ivalice, it's covered in a thick, magical Mist. Generations later, it undergoes a huge Cataclysm that wipes out entire races, ruins the world, and throws it back into the dark ages. We've seen this play out with another world: Gaia of Final Fantasy IX. Perhaps Ivalice is also subject to a forcible realm merger, which ultimately fails and results in the Cataclysm.