Talking to ONM, Motohide Eshiro, producer of Okamiden for DS explains why the game is only bound for that console.

“The distinctive feature of the Celestial Brush is just screaming out to be realised on the DS touch screen. I think it’s only natural that Okamiden was designed for DS. …Showing such diverse, sensitive colours that were seen in the original Okami was one tough task indeed. However, it was rather fortuitous for Capcom to have plenty of talented programmers and graphic designers that they have put everything into this title and recreated that very experience on the DS.”