Well, Star Ocean 4 was a success in Japan and it wa son the XBOX...
Apparently a certain transaction made both Star Ocean 4 and Tales of Vesperia timed exclusive on the 360, but now that both are getting a release on PS3, it's quite telling.
Unfortunately Suikoden isn't as big a franchise, or maybe Konami would have been offered a similar deal.
From what I've read, some of the early exclusive 360 JRPGs were a result of the available devkits when the projects were started. Though I could see the publishers milking this for some money from Microsoft.
Regardless, the era of the 360-exclusive JRPG seems to be over - Magna Carta 2 was the only prominent one for a long time, and I don't know of any in the future. One reason could be the success of PS3 JRPGs (such as Tales of Vesperia PS3 selling more than twice the 360 version, a year later) greatly driving up the price of any exclusivity deals, to the point where it's no longer an attractive prospect for Microsoft.
It would make the most financial sense to make it multi-platform, but JRPG developers don't seem to be as eager as WRPG developers to see their games on multiple platforms. Hopefully Vandal Hearts: Flames of Judgment means that Konami is starting to change their tune, and the game is enough of a success to continue the trend.