I mean...Ivalice had two games that built up the lore and world before XII. Something other mainline games don't get.
If you took XII by itself, the lore and story aren't really all that deep. It is only once you play Vagrant Story and Tactics that you realize the lore is bigger than what XII shows.
This is factually incorrect. And Vagrant Story doesn't even have any bearing on Ivalice lore (from Matsuno's point of view VS isn't even part of Ivalice).
Besides, FFXV had a movie, an anime series and a few spinoffs to build up its lore and world before the main game's release and yet (no offense to FFXV fans but) I don't see many people clamoring for another FF game set in Eos.