I'm thinking why that is is the spoilers and the spoiling news that SE kept making. Since people know what to expect or having a hunch, they decided not to buy it. Of course, other reasons would include financial issues like I didn't get enough money or I need to take care of the rent and bills (I'll be most likely in that situation all thanks to the economic crisis still lurking in the US).
On a positive note, earning small sales is better than not selling a single game at all. We had to wait what the profits will make when February comes around. Nevertheless, I'm sure SE will make a satifiable conclusion of FFXIII and the fans will like it.