Seaman 1 originally came out on the Dreamcast, and it was a very big a well seellign game (in Japan and to a lesser extent Europe). It was more of a pure Niche title in America.
THey announced this game ages ago, I thought it had been cancelled for about a year. They were probably so far into development of the PS2 version that switching it to the PS3 wouldn't be feasable for them on their budget (not huge despite the sucess of Seaman).
I'd imagine a Seaman game will probably appear on the PS3 or Wii (likely) at some point. I also think Sega is stupid for not making a port of Seaman to the DS.