Why? you ask
1.(the main reason) this game and Versus XIII is being design using the white engine which can ONLY run on the PS3 which means that if it goes multiplat Square has to redesign the game for scratch ,which means a waste of a lot of time and money
2.multiplatform don't take advantage of the systems strengths for example , take a look at MGS2:S it was ported to the XBOX and it sucked , that's because the game was designed for the PS2 ,now unless you want FF to turn into a half-assed EA sports game it's better sticks to one console. for example FFXI was ported to the 360 to take advantge of it's online capabilities and the Wii FFCC game is a multiplayer game and it fits the Wii
3.the XBOX360 is doing horribly in Japan (and that's where the majority of the fanbase is) and the Wii can't even handle the trailer
4.the main FFs have always appeared on the system with the most storage space and seeing as this game is going to be very big, the game apearing on a dual-layer blu-ray 50GB disk doesn't seem like a impossible thing
5.Sony's relationship with Square(and most 3rd parties) are very good, they even own 8% of SE stocks
6.There was never a multi platform release of an FF game(remember port=/multiplatform)