I'll tell you why: FFCC: medium-to-high-poly character models w/ hair/clothing physics, complex environment modeling (look at Alfitaria, and inside Moogle houses), fur rendering on characters and environments, detailed liquid refraction/reflection effects thanks to subtle bump-mapping, and LOTS of needless, yet accurate self-shadowing (this i'd say is a biggie) on the boat's sail, from cloud cover, from trees, and on characters. A lot of existing, previously featured technologies were efficiently employed at once while maintaining a stable 30fps framerate. I mention all these things aside from the game's artwork and particularly detailed texturing. Sure, Square has barely gotten into the system, but they did a lot of good things "right" on the first try.
The shifty "I hope I can be" 60fps framerate in Rebel Strike shows Factor5's 3D engine was asking for *too much* in various places, as was NST's engine in 1080*: Avalanche. These were game I thought were better off at 30fps instead of 60.
Movie compression is nothing new. Factor5 just had the good sense to apply DivX; it's the DivX team to thank for the high video quality.