Because they've been unable to get the bat off their shoulder all generation long. But the series has declined rather precipitously since going to handhelds, so as Nomura said some time ago, it's time for Kingdom Hearts to return to home consoles.
The reason it's coming to next gen consoles instead of being a late gen PS360 game is, of course, that they showed it very early and it won't be ready for years. Please be excited for TGS 2013, E3 2014, TGS 2014, E3 2015...
Noo please SE not that again.. But they already showed both early KHIII "rumored" and Versus back then, and still they've become next-gen, don't think the only matter is how it's long for SE to developp games, they have intentionnaly gave the game a rest on consoles. We had plenty of other FF, *lightning says hi*.
Finally i'll just say it's Square Enix, and you can't explain that.