Because, AFAIK, the bundle still costs more than the 120GB model, even if you were to factor in the price of FFXIII to the 120GB model. Some people might find that they don't need the extra space. And although rare, some PS3 owners might not even want FFXIII, or it could be a Christmas gift in which they can purchase FFXIII later. Most shops were sold out of the bundle from what I read so I doubt there were 70-80k unsold bundles.
@patsu, I think the FFXIII PS3 is the same as the regular 250GB but it's obviously white with the picture on it.