You have to understand the current Japanese market. It has been in decline for sometime now. Last years sales were down a lot from the previous year.
This year has pretty much continued that trend.
Last week a total of 117k units were sold. Compared to 168k for the same week the previous year. And compared to 183k for the same week 2 years ago.
This generation probably peaked 2 years agon in Japan and since that time numbers have fallen far. Even the PS3 is heavily down. Last week it sold 25k, the same week last year it sold 60k+.
Square Enix are just doing what other Japanese games companies are having to do which is think about how to continue to make money in the current climate. To do this means they have to appeal more to western gamers to bridge the gap from the falling Japanese numbers.
FFXIII did bad in Japan in comparison to other FF games but in comparison to other japanese releases of the past few years it has done very well. That is the only way you can compare them. The landscape has changed.
