Because everyone thinks it is a spinoff when really the only thing similar in the games is the use of crystals and the universe they are in, everything else is different, plus we don't know that much about versus outside a view CG trailers. TGS and S-E's Japan event in two weeks alot will be revealed on FFXIII, FFvsXIII, FFaXIII and a possible new PS3 exclusive from S-E.
http://www.n4g.com/events_e32008/News-172631.aspx
Complete speculation for now though.
End of 2014 Hardware Predictions (03/03/14)
PlayStation 4: 12-15million
Xbox One: 7-10 million
Wii U: 8-9 million (Changed 01/04/2014 from 7-9 --> 8-9 million)