I think the main reason was because there are so many different dialogue outcomes based on what can you say and how NPCs respond to that. And the MotionScan software takes up a lot of data hence why it can't all fit on one disc. Rockstar claims that the
game was originally going to ship on 6 discs until they started cutting content!
Yeah, it's the motion scan. My concern is that if it took that much space... how will we be able to fully utilize it in the future without sacrificing in the graphics department? Will games eventually need to use 3 BLU-RAY DISKS?