Considering that many upper-class peeps are moving on to farm Extreme Modes now, I suspect this is Square Enix's way of ensuring a good cycle of players still running the older modes, especially to benefit new players who need to clear them for their relics. Personally, I've wanted the Nightmare mount since forever. I saw it only once, during my first few wins. It was the same for the other mounts. I got a feeling that the drop percentarge of these mounts is high when you're starting to farm it. It acts as the 'hook' to get players invested in the fights, all of them thinking they will see it again. I haven't seen that mount drop since my 2nd or 3rd win.

I wish there was a Luck materia, to be honest. Luck materia exists in the other FF games. Why not here? It would act as the materia that increases a chance of a rare item. It would at least make grinding these older modes worth it, creating an incentive for veteran gamers to return to farming hard modes and helping new players running for their relics.