It really comes down to development cycle.
FFXIII has been in development for a very long time - longer than most games this generation. Only Killzone 2 and Alan Wake (I assume) could of been in development longer.
Because of such a long production cycle, Square had to look ahead and decide which system to put it out for. By their logic, they assumed the game would come out in 2009-2010 and by then, the PS3 should have taken #1st place which would have made it a reasonable choice.
But things don't always work out that way. With DQIX, when it began production, Square knew the DS was doing, and going to do rather well, since it was Nintendo's new handheld. With FFXIII, though, they picked the worst console. I believe that is why it is on the 360 now - a year or two into the cycle (probably 2006 or 2007), they realized that the PS3 was going to sell much worse than projected, and the 360 was doing at, or slightly above, expectations (except in Japan).
So what does Square do? Make the logical choice: The 360 was sure to sell better than the PS3 in America, and it was a toss-up in PAL territories once the PS3 launched and initial luster died off. So they dev XIII for the 360 as well (and get a few games out there on the 360 too to ensure their familiarity with the console).
It was a great move by Square, but unfortunately, it hurt them thanks to having such a long development cycle.
Back from the dead, I'm afraid.
