Let me get right off the bat by saying that Final Fantasy XIII-2 hasn't exactly sold badly. On PS3 it sold just a little over 500,000 in less than a week; we're not talking Modern Warfare 3 sales here but it was enough to top the sales charts. The less said about the 360 version of the game (10,000 copies sold…) the better, but the Japanese aren't exactly known for their love of the console that Microsoft built.
It doesn't change the fact that Final Fantasy XIII-2 sold a third of the copies that Final Fantasy XIII did during its first week, the latter selling over 1,500,000 copies after its December 17th 2009 release date. Whether this is a sign of the times or some indication towards the fan's reaction to the direction of the series is difficult to say, but it'll be great fun speculating – especially if you dislike Square's recent efforts.