There's no reason not to. Okami is extremely underrated as is and could benefit from being on the 360 and possibly the Wii U later on to give more people a chance to play it.
In this case, I think it's a matter of cost for Capcom. I don't believe Okami has ever been a financial juggernaut, even considering separate releases for two different consoles. It's being made a digital download only outside Japan likely because they already have little confidence in it selling well. Adding in a 360 version for a decidedly niche game is probably too risky a move in Capcom's eyes.
It's a shame, too. Okami is my favorite game of the entire last generation, and it'd be great for everyone this generation to have a chance to play it. At this point, only 360 owners won't have the option to get it, as it was already released on Wii.
"It means that Sir Issac Newton is the deadliest son of a $%#^ in space."