EA has a deal with N, so Square probably tried to get rid of Sony if that theory is true. Sure, exclusivity ruins their sales. They have like 4 or 5 FF titles +the (S)NES ports on the PS already. Also world+dog releases their RPGs on PS2, I guess the competition just gets too strong, especially with the decline in Square's quality. Maybe they see the untapped markets that are GC and XB.
BTW, why does everyone say Square made FFCC just to have access to the GBA? Don't you think they could make GBA games without developing for GC first? And don't you think FFCC would have been much lower quality if that was true?